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Read Basics of starting java language


If you've never used an object-oriented programming language before, you'll need to learn a few basic concepts before you can begin writing any code. This lesson will introduce you to objects, classes, inheritance, interfaces, and packages. Each discussion focuses on how these concepts relate to the real world, while simultaneously providing an introduction to the syntax of the Java programming language.
What Is an Object?
An object is a software bundle of related state and behavior. Software objects are often used to model the real-world objects that you find in everyday life. This lesson explains how state and behavior are represented within an object, introduces the concept of data encapsulation, and explains the benefits of designing your software in this manner.
What Is a Class?
A class is a blueprint or prototype from which objects are created. This section defines a class that models the state and behavior of a real-world object. It intentionally focuses on the basics, showing how even a simple class can cleanly model state and behavior.
What Is Inheritance?
Inheritance provides a powerful and natural mechanism for organizing and structuring your software. This section explains how classes inherit state and behavior from their superclasses, and explains how to derive one class from another using the simple syntax provided by the Java programming language.
What Is an Interface?
An interface is a contract between a class and the outside world. When a class implements an interface, it promises to provide the behavior published by that interface. This section defines a simple interface and explains the necessary changes for any class that implements it.
What Is a Package?
A package is a namespace for organizing classes and interfaces in a logical manner. Placing your code into packages makes large software projects easier to manage. This section explains why this is useful, and introduces you to the Application Programming Interface (API) provided by the Java platform.
What Is an Object?
Objects are key to understanding object-oriented technology. Look around right now and you'll find many examples of real-world objects: your dog, your desk, your television set, your bicycle.
Real-world objects share two characteristics: They all have state and behavior. Dogs have state (name, color, breed, hungry) and behavior (barking, fetching, wagging tail). Bicycles also have state (current gear, current pedal cadence, current speed) and behavior (changing gear, changing pedal cadence, applying brakes). Identifying the state and behavior for real-world objects is a great way to begin thinking in terms of object-oriented programming.
Take a minute right now to observe the real-world objects that are in your immediate area. For each object that you see, ask yourself two questions: "What possible states can this object be in?" and "What possible behavior can this object perform?". Make sure to write down your observations. As you do, you'll notice that real-world objects vary in complexity; your desktop lamp may have only two possible states (on and off) and two possible behaviors (turn on, turn off), but your desktop radio might have additional states (on, off, current volume, current station) and behavior (turn on, turn off, increase volume, decrease volume, seek, scan, and tune). You may also notice that some objects, in turn, will also contain other objects. These real-world observations all translate into the world of object-oriented programming
What Is a Class?
In the real world, you'll often find many individual objects all of the same kind. There may be thousands of other bicycles in existence, all of the same make and model. Each bicycle was built from the same set of blueprints and therefore contains the same components. In object-oriented terms, we say that your bicycle is an instance of the class of objects known as bicycles. A class is the blueprint from which individual objects are created.
The following Bicycle class is one possible implementation of a bicycle:

class Bicycle {

int cadence = 0;
int speed = 0;
int gear = 1;

void changeCadence(int newValue) {
cadence = newValue;
}

void changeGear(int newValue) {
gear = newValue;
}

void speedUp(int increment) {
speed = speed + increment;
}

void applyBrakes(int decrement) {
speed = speed - decrement;
}

void printStates() {
System.out.println("cadence:"+cadence+" speed:"+speed+" gear:"+gear);
}
}
The syntax of the Java programming language will look new to you, but the design of this class is based on the previous discussion of bicycle objects. The fields cadence, speed, and gear represent the object's state, and the methods (changeCadence, changeGear, speedUp etc.) define its interaction with the outside world.
You may have noticed that the Bicycle class does not contain a main method. That's because it's not a complete application; it's just the blueprint for bicycles that might be used in an application. The responsibility of creating and using new Bicycle objects belongs to some other class in your application.
Here's a BicycleDemo class that creates two separate Bicycle objects and invokes their methods:

class BicycleDemo {
public static void main(String[] args) {

// Create two different Bicycle objects
Bicycle bike1 = new Bicycle();
Bicycle bike2 = new Bicycle();

// Invoke methods on those objects
bike1.changeCadence( 50);
bike1.speedUp(10);
bike1.changeGear(2);
bike1.printStates();

bike2.changeCadence( 50);
bike2.speedUp(10);
bike2.changeGear(2);
bike2.changeCadence( 40);
bike2.speedUp(10);
bike2.changeGear(3);
bike2.printStates();
}
}

The output of this test prints the ending pedal cadence, speed, and gear for the two bicycles:
cadence:50 speed:10 gear:2
cadence:40 speed:20 gear:3

What Is Inheritance?
Different kinds of objects often have a certain amount in common with each other. Mountain bikes, road bikes, and tandem bikes, for example, all share the characteristics of bicycles (current speed, current pedal cadence, current gear). Yet each also defines additional features that make them different: tandem bicycles have two seats and two sets of handlebars; road bikes have drop handlebars; some mountain bikes have an additional chain ring, giving them a lower gear ratio.
Object-oriented programming allows classes to inherit commonly used state and behavior from other classes. In this example, Bicycle now becomes the superclass of MountainBike, RoadBike, and TandemBike. In the Java programming language, each class is allowed to have one direct superclass, and each superclass has the potential for an unlimited number of subclasses:

A hierarchy of bicycle classes.
The syntax for creating a subclass is simple. At the beginning of your class declaration, use the extends keyword, followed by the name of the class to inherit from:
class MountainBike extends Bicycle {

// new fields and methods defining a mountain bike would go here

}
This gives MountainBike all the same fields and methods as Bicycle, yet allows its code to focus exclusively on the features that make it unique. This makes code for your subclasses easy to read. However, you must take care to properly document the state and behavior that each superclass defines, since that code will not appear in the source file of each subclass.

What Is an Interface?
As you've already learned, objects define their interaction with the outside world through the methods that they expose. Methods form the object's interface with the outside world; the buttons on the front of your television set, for example, are the interface between you and the electrical wiring on the other side of its plastic casing. You press the "power" button to turn the television on and off.
In its most common form, an interface is a group of related methods with empty bodies. A bicycle's behavior, if specified as an interface, might appear as follows:
interface Bicycle {

void changeCadence(int newValue);

void changeGear(int newValue);

void speedUp(int increment);

void applyBrakes(int decrement);
}
To implement this interface, the name of your class would change (to ACMEBicycle, for example), and you'd use the implements keyword in the class declaration:
class ACMEBicycle implements Bicycle {

// remainder of this class implemented as before

}
Implementing an interface allows a class to become more formal about the behavior it promises to provide. Interfaces form a contract between the class and the outside world, and this contract is enforced at build time by the compiler. If your class claims to implement an interface, all methods defined by that interface must appear in its source code before the class will successfully compile.

What Is a Package?
A package is a namespace that organizes a set of related classes and interfaces. Conceptually you can think of packages as being similar to different folders on your computer. You might keep HTML pages in one folder, images in another, and scripts or applications in yet another. Because software written in the Java programming language can be composed of hundreds or thousands of individual classes, it makes sense to keep things organized by placing related classes and interfaces into packages.
The Java platform provides an enormous class library (a set of packages) suitable for use in your own applications. This library is known as the "Application Programming Interface", or "API" for short. Its packages represent the tasks most commonly associated with general-purpose programming. For example, a String object contains state and behavior for character strings; a File object allows a programmer to easily create, delete, inspect, compare, or modify a file on the filesystem; a Socket object allows for the creation and use of network sockets; various GUI objects control buttons and checkboxes and anything else related to graphical user interfaces. There are literally thousands of classes to choose from. This allows you, the programmer, to focus on the design of your particular application, rather than the infrastructure required to make it work.
The Java Platform API Specification contains the complete listing for all packages, interfaces, classes, fields, and methods supplied by the Java Platform 6, Standard Edition. Load the page in your browser and bookmark it. As a programmer, it will become your single most important piece of reference documentation.
Variables
You've already learned that objects store their state in fields. However, the Java programming language also uses the term "variable" as well. This section discusses this relationship, plus variable naming rules and conventions, basic data types (primitive types, character strings, and arrays), default values, and literals.
Operators
This section describes the operators of the Java programming language. It presents the most commonly-used operators first, and the less commonly-used operators last. Each discussion includes code samples that you can compile and run.
Expressions, Statements, and Blocks
Operators may be used in building expressions, which compute values; expressions are the core components of statements; statements may be grouped into blocks. This section discusses expressions, statements, and blocks using example code that you've already seen.
Control Flow Statements
This section describes the control flow statements supported by the Java programming language. It covers the decisions-making, looping, and branching statements that enable your programs to conditionally execute particular blocks of code

Variables
As you learned in the previous lesson, an object stores its state in fields.
int cadence = 0;
int speed = 0;
int gear = 1;
The What Is an Object? discussion introduced you to fields, but you probably have still a few questions, such as: What are the rules and conventions for naming a field? Besides int, what other data types are there? Do fields have to be initialized when they are declared? Are fields assigned a default value if they are not explicitly initialized? We'll explore the answers to such questions in this lesson, but before we do, there are a few technical distinctions you must first become aware of. In the Java programming language, the terms "field" and "variable" are both used; this is a common source of confusion among new developers, since both often seem to refer to the same thing.
The Java programming language defines the following kinds of variables:
• Instance Variables (Non-Static Fields) Technically speaking, objects store their individual states in "non-static fields", that is, fields declared without the static keyword. Non-static fields are also known as instance variables because their values are unique to each instance of a class (to each object, in other words); the currentSpeed of one bicycle is independent from the currentSpeed of another.
• Class Variables (Static Fields) A class variable is any field declared with the static modifier; this tells the compiler that there is exactly one copy of this variable in existence, regardless of how many times the class has been instantiated. A field defining the number of gears for a particular kind of bicycle could be marked as static since conceptually the same number of gears will apply to all instances. The code static int numGears = 6; would create such a static field. Additionally, the keyword final could be added to indicate that the number of gears will never change.
• Local Variables Similar to how an object stores its state in fields, a method will often store its temporary state in local variables. The syntax for declaring a local variable is similar to declaring a field (for example, int count = 0. There is no special keyword designating a variable as local; that determination comes entirely from the location in which the variable is declared — which is between the opening and closing braces of a method. As such, local variables are only visible to the methods in which they are declared; they are not accessible from the rest of the class.
• Parameters You've already seen examples of parameters, both in the Bicycle class and in the main method of the "Hello World!" application. Recall that the signature for the main method is public static void main(String[] args). Here, the args variable is the parameter to this method. The important thing to remember is that parameters are always classified as "variables" not "fields". This applies to other parameter-accepting constructs as well (such as constructors and exception handlers) that you'll learn about later in the tutorial.
Having said that, the remainder of this tutorial uses the following general guidelines when discussing fields and variables. If we are talking about "fields in general" (excluding local variables and parameters), we may simply say "fields". If the discussion applies to "all of the above", we may simply say "variables". If the context calls for a distinction, we will use specific terms (static field, local variables, etc.) as appropriate. You may also occasionally see the term "member" used as well. A type's fields, methods, and nested types are collectively called its members.
Naming
Every programming language has its own set of rules and conventions for the kinds of names that you're allowed to use, and the Java programming language is no different. The rules and conventions for naming your variables can be summarized as follows:
• Variable names are case-sensitive. A variable's name can be any legal identifier — an unlimited-length sequence of Unicode letters and digits, beginning with a letter, the dollar sign "$", or the underscore character "_". The convention, however, is to always begin your variable names with a letter, not "$" or "_". Additionally, the dollar sign character, by convention, is never used at all. You may find some situations where auto-generated names will contain the dollar sign, but your variable names should always avoid using it. A similar convention exists for the underscore character; while it's technically legal to begin your variable's name with "_", this practice is discouraged. White space is not permitted.
• Subsequent characters may be letters, digits, dollar signs, or underscore characters. Conventions (and common sense) apply to this rule as well. When choosing a name for your variables, use full words instead of cryptic abbreviations. Doing so will make your code easier to read and understand. In many cases it will also make your code self-documenting; fields named cadence, speed, and gear, for example, are much more intuitive than abbreviated versions, such as s, c, and g. Also keep in mind that the name you choose must not be a keyword or reserved word.
• If the name you choose consists of only one word, spell that word in all lowercase letters. If it consists of more than one word, capitalize the first letter of each subsequent word. The names gearRatio and currentGear are prime examples of this convention. If your variable stores a constant value, such as static final int NUM_GEARS = 6, the convention changes slightly, capitalizing every letter and separating subsequent words with the underscore character. By convention, the underscore character is never used elsewhere.
Primitive Data Types
The Java programming language is strongly-typed, which means that all variables must first be declared before they can be used. This involves stating the variable's type and name, as you've already seen:
int gear = 1;
Doing so tells your program that a field named "gear" exists, holds numerical data, and has an initial value of "1". A variable's data type determines the values it may contain, plus the operations that may be performed on it. In addition to int, the Java programming language supports seven other primitive data types. A primitive type is predefined by the language and is named by a reserved keyword. Primitive values do not share state with other primitive values. The eight primitive data types supported by the Java programming language are:
• byte: The byte data type is an 8-bit signed two's complement integer. It has a minimum value of -128 and a maximum value of 127 (inclusive). The byte data type can be useful for saving memory in large arrays, where the memory savings actually matters. They can also be used in place of int where their limits help to clarify your code; the fact that a variable's range is limited can serve as a form of documentation.
• short: The short data type is a 16-bit signed two's complement integer. It has a minimum value of -32,768 and a maximum value of 32,767 (inclusive). As with byte, the same guidelines apply: you can use a short to save memory in large arrays, in situations where the memory savings actually matters.
• int: The int data type is a 32-bit signed two's complement integer. It has a minimum value of -2,147,483,648 and a maximum value of 2,147,483,647 (inclusive). For integral values, this data type is generally the default choice unless there is a reason (like the above) to choose something else. This data type will most likely be large enough for the numbers your program will use, but if you need a wider range of values, use long instead.
• long: The long data type is a 64-bit signed two's complement integer. It has a minimum value of -9,223,372,036,854,77 5,808 and a maximum value of 9,223,372,036,854,77 5,807 (inclusive). Use this data type when you need a range of values wider than those provided by int.
• float: The float data type is a single-precision 32-bit IEEE 754 floating point. Its range of values is beyond the scope of this discussion, but is specified in section 4.2.3 of the Java Language Specification. As with the recommendations for byte and short, use a float (instead of double) if you need to save memory in large arrays of floating point numbers. This data type should never be used for precise values, such as currency. For that, you will need to use the java.math.BigDecimal class instead. Numbers and Strings covers BigDecimal and other useful classes provided by the Java platform.
• double: The double data type is a double-precision 64-bit IEEE 754 floating point. Its range of values is beyond the scope of this discussion, but is specified in section 4.2.3 of the Java Language Specification. For decimal values, this data type is generally the default choice. As mentioned above, this data type should never be used for precise values, such as currency.
• boolean: The boolean data type has only two possible values: true and false. Use this data type for simple flags that track true/false conditions. This data type represents one bit of information, but its "size" isn't something that's precisely defined.
• char: The char data type is a single 16-bit Unicode character. It has a minimum value of '\u0000' (or 0) and a maximum value of '\uffff' (or 65,535 inclusive).
In addition to the eight primitive data types listed above, the Java programming language also provides special support for character strings via the java.lang.String class. Enclosing your character string within double quotes will automatically create a new String object; for example, String s = "this is a string";. String objects are immutable, which means that once created, their values cannot be changed. The String class is not technically a primitive data type, but considering the special support given to it by the language, you'll probably tend to think of it as such. You'll learn more about the String class in Simple Data Objects
Default Values
It's not always necessary to assign a value when a field is declared. Fields that are declared but not initialized will be set to a reasonable default by the compiler. Generally speaking, this default will be zero or null, depending on the data type. Relying on such default values, however, is generally considered bad programming style.
The following chart summarizes the default values for the above data types.
Data Type Default Value (for fields)
byte 0
short 0
int 0
long 0L
float 0.0f
double 0.0d
char '\u0000'
String (or any object) null
boolean false
Local variables are slightly different; the compiler never assigns a default value to an uninitialized local variable. If you cannot initialize your local variable where it is declared, make sure to assign it a value before you attempt to use it. Accessing an uninitialized local variable will result in a compile-time error.
Literals
You may have noticed that the new keyword isn't used when initializing a variable of a primitive type. Primitive types are special data types built into the language; they are not objects created from a class. A literal is the source code representation of a fixed value; literals are represented directly in your code without requiring computation. As shown below, it's possible to assign a literal to a variable of a primitive type:
boolean result = true;
char capitalC = 'C';
byte b = 100;
short s = 10000;
int i = 100000;
The integral types (byte, short, int, and long) can be expressed using decimal, octal, or hexadecimal number systems. Decimal is the number system you already use every day; it's based on 10 digits, numbered 0 through 9. The octal number system is base 8, consisting of the digits 0 through 7. The hexadecimal system is base 16, whose digits are the numbers 0 through 9 and the letters A through F. For general-purpose programming, the decimal system is likely to be the only number system you'll ever use. However, if you need octal or hexadecimal, the following example shows the correct syntax. The prefix 0 indicates octal, whereas 0x indicates hexadecimal.
int decVal = 26; // The number 26, in decimal
int octVal = 032; // The number 26, in octal
int hexVal = 0x1a; // The number 26, in hexadecimal
The floating point types (float and double) can also be expressed using E or e (for scientific notation), F or f (32-bit float literal) and D or d (64-bit double literal; this is the default and by convention is omitted).
double d1 = 123.4;
double d2 = 1.234e2; // same value as d1, but in scientific notation
float f1 = 123.4f;
Literals of types char and String may contain any Unicode (UTF-16) characters. If your editor and file system allow it, you can use such characters directly in your code. If not, you can use a "Unicode escape" such as '\u0108' (capital C with circumflex), or "S\u00ED se\u00F1or" (Sí Señor in Spanish). Always use 'single quotes' for char literals and "double quotes" for String literals. Unicode escape sequences may be used elsewhere in a program (such as in field names, for example), not just in char or String literals.
The Java programming language also supports a few special escape sequences for char and String literals: \b (backspace), \t (tab), \n (line feed), \f (form feed), \r (carriage return), \" (double quote), \' (single quote), and \\ (backslash).
There's also a special null literal that can be used as a value for any reference type. null may be assigned to any variable, except variables of primitive types. There's little you can do with a null value beyond testing for its presence. Therefore, null is often used in programs as a marker to indicate that some object is unavailable.
Finally, there's also a special kind of literal called a class literal, formed by taking a type name and appending ".class"; for example, String.class. This refers to the object (of type Class) that represents the type itself.
Arrays
An array is a container object that holds a fixed number of values of a single type. The length of an array is established when the array is created. After creation, its length is fixed. You've seen an example of arrays already, in the main method of the "Hello World!" application. This section discusses arrays in greater detail.

An array of ten elements
Each item in an array is called an element, and each element is accessed by its numerical index. As shown in the above illustration, numbering begins with 0. The 9th element, for example, would therefore be accessed at index 8.
The following program, ArrayDemo, creates an array of integers, puts some values in it, and prints each value to standard output.

class ArrayDemo {
public static void main(String[] args) {
int[] anArray; // declares an array of integers

anArray = new int[10]; // allocates memory for 10 integers

anArray[0] = 100; // initialize first element
anArray[1] = 200; // initialize second element
anArray[2] = 300; // etc.
anArray[3] = 400;
anArray[4] = 500;
anArray[5] = 600;
anArray[6] = 700;
anArray[7] = 800;
anArray[8] = 900;
anArray[9] = 1000;

System.out.println("Element at index 0: " + anArray[0]);
System.out.println("Element at index 1: " + anArray[1]);
System.out.println("Element at index 2: " + anArray[2]);
System.out.println("Element at index 3: " + anArray[3]);
System.out.println("Element at index 4: " + anArray[4]);
System.out.println("Element at index 5: " + anArray[5]);
System.out.println("Element at index 6: " + anArray[6]);
System.out.println("Element at index 7: " + anArray[7]);
System.out.println("Element at index 8: " + anArray[8]);
System.out.println("Element at index 9: " + anArray[9]);
}
}
The output from this program is:
Element at index 0: 100
Element at index 1: 200
Element at index 2: 300
Element at index 3: 400
Element at index 4: 500
Element at index 5: 600
Element at index 6: 700
Element at index 7: 800
Element at index 8: 900
Element at index 9: 1000
In a real-world programming situation, you'd probably use one of the supported looping constructs to iterate through each element of the array, rather than write each line individually as shown above. However, this example clearly illustrates the array syntax. You'll learn about the various looping constructs (for, while, and do-while) in the Control Flow section.
Declaring a Variable to Refer to an Array
The above program declares anArray with the following line of code:
int[] anArray; // declares an array of integers
Like declarations for variables of other types, an array declaration has two components: the array's type and the array's name. An array's type is written as type[], where type is the data type of the contained elements; the square brackets are special symbols indicating that this variable holds an array. The size of the array is not part of its type (which is why the brackets are empty). An array's name can be anything you want, provided that it follows the rules and conventions as previously discussed in the naming section. As with variables of other types, the declaration does not actually create an array — it simply tells the compiler that this variable will hold an array of the specified type.
Similarly, you can declare arrays of other types:
byte[] anArrayOfBytes;
short[] anArrayOfShorts;
long[] anArrayOfLongs;
float[] anArrayOfFloats;
double[] anArrayOfDoubles;
boolean[] anArrayOfBooleans;
char[] anArrayOfChars;
String[] anArrayOfStrings;
You can also place the square brackets after the array's name:
float anArrayOfFloats[]; // this form is discouraged
However, convention discourages this form; the brackets identify the array type and should appear with the type designation.
Creating, Initializing, and Accessing an Array
One way to create an array is with the new operator. The next statement in the ArrayDemo program allocates an array with enough memory for ten integer elements and assigns the array to the anArray variable.
anArray = new int[10]; // create an array of integers
If this statement were missing, the compiler would print an error like the following, and compilation would fail:
ArrayDemo.java Variable anArray may not have been initialized.
The next few lines assign values to each element of the array:
anArray[0] = 100; // initialize first element
anArray[1] = 200; // initialize second element
anArray[2] = 300; // etc.
Each array element is accessed by its numerical index:
System.out.println("Element 1 at index 0: " + anArray[0]);
System.out.println("Element 2 at index 1: " + anArray[1]);
System.out.println("Element 3 at index 2: " + anArray[2]);
Alternatively, you can use the shortcut syntax to create and initialize an array:
int[] anArray = {100, 200, 300, 400, 500, 600, 700, 800, 900, 1000};
Here the length of the array is determined by the number of values provided between { and }.
You can also declare an array of arrays (also known as a multidimensional array) by using two or more sets of square brackets, such as String[][] names. Each element, therefore, must be accessed by a corresponding number of index values.
In the Java programming language, a multidimensional array is simply an array whose components are themselves arrays. This is unlike arrays in C or Fortran. A consequence of this is that the rows are allowed to vary in length, as shown in the following MultiDimArrayDemo program:
class MultiDimArrayDemo {
public static void main(String[] args) {
String[][] names = {{"Mr. ", "Mrs. ", "Ms. "},
{"Smith", "Jones"}};
System.out.println(n ames[0][0] + names[1][0]); //Mr. Smith
System.out.println(n ames[0][2] + names[1][1]); //Ms. Jones
}
}
The output from this program is:
Mr. Smith
Ms. Jones
Finally, you can use the built-in length property to determine the size of any array. The code
System.out.println(a nArray.length);
will print the array's size to standard output.
Copying Arrays
The System class has an arraycopy method that you can use to efficiently copy data from one array into another:
public static void arraycopy(Object src,
int srcPos,
Object dest,
int destPos,
int length)
The two Object arguments specify the array to copy from and the array to copy to. The three int arguments specify the starting position in the source array, the starting position in the destination array, and the number of array elements to copy.
The following program, ArrayCopyDemo, declares an array of char elements, spelling the word "decaffeinated". It uses arraycopy to copy a subsequence of array components into a second array:

class ArrayCopyDemo {
public static void main(String[] args) {
char[] copyFrom = { 'd', 'e', 'c', 'a', 'f', 'f', 'e',
'i', 'n', 'a', 't', 'e', 'd' };
char[] copyTo = new char[7];

System.arraycopy(cop yFrom, 2, copyTo, 0, 7);
System.out.println(n ew String(copyTo));
}
}
The output from this program is:
caffein

Summary of Variables
The Java programming language uses both "fields" and "variables" as part of its terminology. Instance variables (non-static fields) are unique to each instance of a class. Class variables (static fields) are fields declared with the static modifier; there is exactly one copy of a class variable, regardless of how many times the class has been instantiated. Local variables store temporary state inside a method. Parameters are variables that provide extra information to a method; both local variables and parameters are always classified as "variables" (not "fields"). When naming your fields or variables, there are rules and conventions that you should (or must) follow.
The eight primitive data types are: byte, short, int, long, float, double, boolean, and char. The java.lang.String class represents character strings. The compiler will assign a reasonable default value for fields of the above types; for local variables, a default value is never assigned. A literal is the source code representation of a fixed value. An array is a container object that holds a fixed number of values of a single type. The length of an array is established when the array is created. After creation, its length is fixed.
Operators
Now that you've learned how to declare and initialize variables, you probably want to know how to do something with them. Learning the operators of the Java programming language is a good place to start. Operators are special symbols that perform specific operations on one, two, or three operands, and then return a result.
As we explore the operators of the Java programming language, it may be helpful for you to know ahead of time which operators have the highest precedence. The operators in the following table are listed according to precedence order. The closer to the top of the table an operator appears, the higher its precedence. Operators with higher precedence are evaluated before operators with relatively lower precedence. Operators on the same line have equal precedence. When operators of equal precedence appear in the same expression, a rule must govern which is evaluated first. All binary operators except for the assignment operators are evaluated from left to right; assignment operators are evaluated right to left.
Operator Precedence
Operators Precedence
postfix expr++ expr--
unary ++expr --expr +expr -expr ~ !
multiplicative * / %
additive + -
shift << >> >>>
relational < > <= >= instanceof
equality == !=
bitwise AND &
bitwise exclusive OR ^
bitwise inclusive OR |
logical AND &&
logical OR ||
ternary ? :
assignment = += -= *= /= %= &= ^= |= <<= >>= >>>=
In general-purpose programming, certain operators tend to appear more frequently than others; for example, the assignment operator "=" is far more common than the unsigned right shift operator ">>>". With that in mind, the following discussion focuses first on the operators that you're most likely to use on a regular basis, and ends focusing on those that are less common. Each discussion is accompanied by sample code that you can compile and run. Studying its output will help reinforce what you've just learned.
Assignment, Arithmetic, and Unary Operators
The Simple Assignment Operator
One of the most common operators that you'll encounter is the simple assignment operator "=". You saw this operator in the Bicycle class; it assigns the value on its right to the operand on its left:
int cadence = 0;
int speed = 0;
int gear = 1;
This operator can also be used on objects to assign object references, as discussed in Creating Objects.
The Arithmetic Operators
The Java programming language provides operators that perform addition, subtraction, multiplication, and division. There's a good chance you'll recognize them by their counterparts in basic mathematics. The only symbol that might look new to you is "%", which divides one operand by another and returns the remainder as its result.
+ additive operator (also used for String concatenation)
- subtraction operator
* multiplication operator
/ division operator
% remainder operator
The following program, ArithmeticDemo, tests the arithmetic operators.

class ArithmeticDemo {

public static void main (String[] args){

int result = 1 + 2; // result is now 3
System.out.println(r esult);

result = result - 1; // result is now 2
System.out.println(r esult);

result = result * 2; // result is now 4
System.out.println(r esult);

result = result / 2; // result is now 2
System.out.println(r esult);

result = result + 8; // result is now 10
result = result % 7; // result is now 3
System.out.println(r esult);

}
}
You can also combine the arithmetic operators with the simple assignment operator to create compound assignments. For example, x+=1; and x=x+1; both increment the value of x by 1.
The + operator can also be used for concatenating (joining) two strings together, as shown in the following ConcatDemo program:

class ConcatDemo {
public static void main(String[] args){
String firstString = "This is";
String secondString = " a concatenated string.";
String thirdString = firstString+secondSt ring;
System.out.println(t hirdString);
}
}
By the end of this program, the variable thirdString contains "This is a concatenated string.", which gets printed to standard output.
The Unary Operators
The unary operators require only one operand; they perform various operations such as incrementing/decrementing a value by one, negating an expression, or inverting the value of a boolean.
+ Unary plus operator; indicates positive value (numbers are positive without this, however)
- Unary minus operator; negates an expression
++ Increment operator; increments a value by 1
-- Decrement operator; decrements a value by 1
! Logical complement operator; inverts the value of a boolean
The following program, UnaryDemo, tests the unary operators:

class UnaryDemo {

public static void main(String[] args){
int result = +1; // result is now 1
System.out.println(r esult);
result--; // result is now 0
System.out.println(r esult);
result+ // result is now 1
System.out.println(r esult);
result = -result; // result is now -1
System.out.println(r esult);
boolean success = false;
System.out.println(s uccess); // false
System.out.println(! success); // true
}
}
The increment/decrement operators can be applied before (prefix) or after (postfix) the operand. The code result+ and ++result; will both end in result being incremented by one. The only difference is that the prefix version (++result) evaluates to the incremented value, whereas the postfix version (result+ evaluates to the original value. If you are just performing a simple increment/decrement, it doesn't really matter which version you choose. But if you use this operator in part of a larger expression, the one that you choose may make a significant difference.
The following program, PrePostDemo, illustrates the prefix/postfix unary increment operator:

class PrePostDemo {
public static void main(String[] args){
int i = 3;
i+
System.out.println(i ); // "4"
++i;
System.out.println(i ); // "5"
System.out.println(+ +i); // "6"
System.out.println(i +; // "6"
System.out.println(i ); // "7"
}
}
Equality, Relational, and Conditional Operators
The Equality and Relational Operators
The equality and relational operators determine if one operand is greater than, less than, equal to, or not equal to another operand. The majority of these operators will probably look familiar to you as well. Keep in mind that you must use "==", not "=", when testing if two primitive values are equal.
== equal to
!= not equal to
> greater than
>= greater than or equal to
< less than
<= less than or equal to
The following program, ComparisonDemo, tests the comparison operators:

class ComparisonDemo {

public static void main(String[] args){
int value1 = 1;
int value2 = 2;
if(value1 == value2) System.out.println("value1 == value2");
if(value1 != value2) System.out.println("value1 != value2");
if(value1 > value2) System.out.println("value1 > value2");
if(value1 < value2) System.out.println("value1 < value2");
if(value1 <= value2) System.out.println("value1 <= value2");
}
}
Output:
value1 != value2
value1 < value2
value1 <= value2
The Conditional Operators
The && and || operators perform Conditional-AND and Conditional-OR operations on two boolean expressions. These operators exhibit "short-circuiting" behavior, which means that the second operand is evaluated only if needed.
&& Conditional-AND
|| Conditional-OR
The following program, ConditionalDemo1, tests these operators:

class ConditionalDemo1 {

public static void main(String[] args){
int value1 = 1;
int value2 = 2;
if((value1 == 1) && (value2 == 2)) System.out.println("value1 is 1 AND value2 is 2");
if((value1 == 1) || (value2 == 1)) System.out.println("value1 is 1 OR value2 is 1");

}
}
Another conditional operator is ?:, which can be thought of as shorthand for an if-then-else statement (discussed in the Control Flow Statements section of this lesson). This operator is also known as the ternary operator because it uses three operands. In the following example, this operator should be read as: "If someCondition is true, assign the value of value1 to result. Otherwise, assign the value of value2 to result."
The following program, ConditionalDemo2, tests the ?: operator:

class ConditionalDemo2 {

public static void main(String[] args){
int value1 = 1;
int value2 = 2;
int result;
boolean someCondition = true;
result = someCondition ? value1 : value2;

System.out.println(r esult);

}
}
Because someCondition is true, this program prints "1" to the screen. Use the ?: operator instead of an if-then-else statement if it makes your code more readable; for example, when the expressions are compact and without side-effects (such as assignments).
The Type Comparison Operator instanceof
The instanceof operator compares an object to a specified type. You can use it to test if an object is an instance of a class, an instance of a subclass, or an instance of a class that implements a particular interface.
The following program, InstanceofDemo, defines a parent class (named Parent), a simple interface (named MyInterface), and a child class (named Child) that inherits from the parent and implements the interface.

class InstanceofDemo {
public static void main(String[] args) {

Parent obj1 = new Parent();
Parent obj2 = new Child();

System.out.println("obj1 instanceof Parent: " + (obj1 instanceof Parent));
System.out.println("obj1 instanceof Child: " + (obj1 instanceof Child));
System.out.println("obj1 instanceof MyInterface: " + (obj1 instanceof MyInterface));
System.out.println("obj2 instanceof Parent: " + (obj2 instanceof Parent));
System.out.println("obj2 instanceof Child: " + (obj2 instanceof Child));
System.out.println("obj2 instanceof MyInterface: " + (obj2 instanceof MyInterface));
}
}

class Parent{}
class Child extends Parent implements MyInterface{}
interface MyInterface{}
Output:
obj1 instanceof Parent: true
obj1 instanceof Child: false
obj1 instanceof MyInterface: false
obj2 instanceof Parent: true
obj2 instanceof Child: true
obj2 instanceof MyInterface: true
When using the instanceof operator, keep in mind that null is not an instance of anything.

Summary of Operators
The following quick reference summarizes the operators supported by the Java programming language.
Simple Assignment Operator
= Simple assignment operator
Arithmetic Operators
+ Additive operator (also used for String concatenation)
- Subtraction operator
* Multiplication operator
/ Division operator
% Remainder operator
Unary Operators
+ Unary plus operator; indicates positive value (numbers are positive without this, however)
- Unary minus operator; negates an expression
++ Increment operator; increments a value by 1
-- Decrement operator; decrements a value by 1
! Logical compliment operator; inverts the value of a boolean
Equality and Relational Operators
== Equal to
!= Not equal to
> Greater than
>= Greater than or equal to
< Less than
<= Less than or equal to
Conditional Operators
&& Conditional-AND
|| Conditional-OR
?: Ternary (shorthand for if-then-else statement)
Type Comparison Operator
instanceof Compares an object to a specified type
Bitwise and Bit Shift Operators
~ Unary bitwise complement
<< Signed left shift
>> Signed right shift
>>> Unsigned right shift
& Bitwise AND
^ Bitwise exclusive OR
| Bitwise inclusive OR

Expressions, Statements, and Blocks
Now that you understand variables and operators, it's time to learn about expressions, statements, and blocks. Operators may be used in building expressions, which compute values; expressions are the core components of statements; statements may be grouped into blocks.
Expressions
An expression is a construct made up of variables, operators, and method invocations, which are constructed according to the syntax of the language, that evaluates to a single value. You've already seen examples of expressions, illustrated in bold below:
int cadence = 0;
anArray[0] = 100;
System.out.println("Element 1 at index 0: " + anArray[0]);

int result = 1 + 2; // result is now 3
if(value1 == value2) System.out.println("value1 == value2");
The data type of the value returned by an expression depends on the elements used in the expression. The expression cadence = 0 returns an int because the assignment operator returns a value of the same data type as its left-hand operand; in this case, cadence is an int. As you can see from the other expressions, an expression can return other types of values as well, such as boolean or String.
The Java programming language allows you to construct compound expressions from various smaller expressions as long as the data type required by one part of the expression matches the data type of the other. Here's an example of a compound expression:

1 * 2 * 3
In this particular example, the order in which the expression is evaluated is unimportant because the result of multiplication is independent of order; the outcome is always the same, no matter in which order you apply the multiplications. However, this is not true of all expressions. For example, the following expression gives different results, depending on whether you perform the addition or the division operation first:
x + y / 100 // ambiguous
You can specify exactly how an expression will be evaluated using balanced parenthesis: ( and ). For example, to make the previous expression unambiguous, you could write the following:

(x + y) / 100 // unambiguous, recommended
If you don't explicitly indicate the order for the operations to be performed, the order is determined by the precedence assigned to the operators in use within the expression. Operators that have a higher precedence get evaluated first. For example, the division operator has a higher precedence than does the addition operator. Therefore, the following two statements are equivalent:
x + y / 100


x + (y / 100) // unambiguous, recommended
When writing compound expressions, be explicit and indicate with parentheses which operators should be evaluated first. This practice makes code easier to read and to maintain.
Statements
Statements are roughly equivalent to sentences in natural languages. A statement forms a complete unit of execution. The following types of expressions can be made into a statement by terminating the expression with a semicolon (.
• Assignment expressions
• Any use of ++ or --
• Method invocations
• Object creation expressions
Such statements are called expression statements. Here are some examples of expression statements.
aValue = 8933.234; // assignment statement
aValue+ // increment statement
System.out.println("Hello World!"); // method invocation statement
Bicycle myBike = new Bicycle(); // object creation statement
In addition to expression statements, there are two other kinds of statements: declaration statements and control flow statements. A declaration statement declares a variable. You've seen many examples of declaration statements already:
double aValue = 8933.234; //declaration statement
Finally, control flow statements regulate the order in which statements get executed. You'll learn about control flow statements in the next section, Control Flow Statements
Blocks
A block is a group of zero or more statements between balanced braces and can be used anywhere a single statement is allowed. The following example, BlockDemo, illustrates the use of blocks:
class BlockDemo {
public static void main(String[] args) {
boolean condition = true;
if (condition) { // begin block 1
System.out.println("Condition is true.");
} // end block one
else { // begin block 2
System.out.println("Condition is false.");
} // end block 2
}
}
The if-then and if-then-else Statements
The if-then Statement
The if-then statement is the most basic of all the control flow statements. It tells your program to execute a certain section of code only if a particular test evaluates to true. For example, the Bicycle class could allow the brakes to decrease the bicycle's speed only if the bicycle is already in motion. One possible implementation of the applyBrakes method could be as follows:
void applyBrakes(){
if (isMoving){ // the "if" clause: bicycle must be moving
currentSpeed--; // the "then" clause: decrease current speed
}
}
If this test evaluates to false (meaning that the bicycle is not in motion), control jumps to the end of the if-then statement.
In addition, the opening and closing braces are optional, provided that the "then" clause contains only one statement:
void applyBrakes(){
if (isMoving) currentSpeed--; // same as above, but without braces
}
Deciding when to omit the braces is a matter of personal taste. Omitting them can make the code more brittle. If a second statement is later added to the "then" clause, a common mistake would be forgetting to add the newly required braces. The compiler cannot catch this sort of error; you'll just get the wrong results.
The if-then-else Statement
The if-then-else statement provides a secondary path of execution when an "if" clause evaluates to false. You could use an if-then-else statement in the applyBrakes method to take some action if the brakes are applied when the bicycle is not in motion. In this case, the action is to simply print an error message stating that the bicycle has already stopped.
void applyBrakes(){
if (isMoving) {
currentSpeed--;
} else {
System.err.println("The bicycle has already stopped!");
}
}
The following program, IfElseDemo, assigns a grade based on the value of a test score: an A for a score of 90% or above, a B for a score of 80% or above, and so on.

class IfElseDemo {
public static void main(String[] args) {

int testscore = 76;
char grade;

if (testscore >= 90) {
grade = 'A';
} else if (testscore >= 80) {
grade = 'B';
} else if (testscore >= 70) {
grade = 'C';
} else if (testscore >= 60) {
grade = 'D';
} else {
grade = 'F';
}
System.out.println("Grade = " + grade);
}
}
The output from the program is:
Grade = C
You may have noticed that the value of testscore can satisfy more than one expression in the compound statement: 76 >= 70 and 76 >= 60. However, once a condition is satisfied, the appropriate statements are executed (grade = 'C' and the remaining conditions are not evaluated.
The switch Statement
Unlike if-then and if-then-else, the switch statement allows for any number of possible execution paths. A switch works with the byte, short, char, and int primitive data types. It also works with enumerated types (discussed in Classes and Inheritance) and a few special classes that "wrap" certain primitive types: Character, Byte, Short, and Integer (discussed in Simple Data Objects ).
The following program, SwitchDemo, declares an int named month whose value represents a month out of the year. The program displays the name of the month, based on the value of month, using the switch statement.

class SwitchDemo {
public static void main(String[] args) {

int month = 8;
switch (month) {
case 1: System.out.println("January"); break;
case 2: System.out.println("February"); break;
case 3: System.out.println("March"); break;
case 4: System.out.println("April"); break;
case 5: System.out.println("May"); break;
case 6: System.out.println("June"); break;
case 7: System.out.println("July"); break;
case 8: System.out.println("August"); break;
case 9: System.out.println("September"); break;
case 10: System.out.println("October"); break;
case 11: System.out.println("November"); break;
case 12: System.out.println("December"); break;
default: System.out.println("Invalid month.");break;
}
}
}
In this case, "August" is printed to standard output.
The body of a switch statement is known as a switch block. Any statement immediately contained by the switch block may be labeled with one or more case or default labels. The switch statement evaluates its expression and executes the appropriate case.
Of course, you could also implement the same thing with if-then-else statements:
int month = 8;
if (month == 1) {
System.out.println("January");
} else if (month == 2) {
System.out.println("February");
}
. . . // and so on
Deciding whether to use if-then-else statements or a switch statement is sometimes a judgment call. You can decide which one to use based on readability and other factors. An if-then-else statement can be used to make decisions based on ranges of values or conditions, whereas a switch statement can make decisions based only on a single integer or enumerated value.
Another point of interest is the break statement after each case. Each break statement terminates the enclosing switch statement. Control flow continues with the first statement following the switch block. The break statements are necessary because without them, case statements fall through; that is, without an explicit break, control will flow sequentially through subsequent case statements. The following program, SwitchDemo2, illustrates why it might be useful to have case statements fall through:

class SwitchDemo2 {
public static void main(String[] args) {

int month = 2;
int year = 2000;
int numDays = 0;

switch (month) {
case 1:
case 3:
case 5:
case 7:
case 8:
case 10:
case 12:
numDays = 31;
break;
case 4:
case 6:
case 9:
case 11:
numDays = 30;
break;
case 2:
if ( ((year % 4 == 0) && !(year % 100 == 0))
|| (year % 400 == 0) )
numDays = 29;
else
numDays = 28;
break;
default:
System.out.println("Invalid month.");
break;
}
System.out.println("Number of Days = " + numDays);
}
}
This is the output from the program.
Number of Days = 29
Technically, the final break is not required because flow would fall out of the switch statement anyway. However, we recommend using a break so that modifying the code is easier and less error-prone. The default section handles all values that aren't explicitly handled by one of the case sections.

The while and do-while Statements
The while statement continually executes a block of statements while a particular condition is true. Its syntax can be expressed as:
while (expression) {
statement(s)
}
The while statement evaluates expression, which must return a boolean value. If the expression evaluates to true, the while statement executes the statement(s) in the while block. The while statement continues testing the expression and executing its block until the expression evaluates to false. Using the while statement to print the values from 1 through 10 can be accomplished as in the following WhileDemo program:

class WhileDemo {
public static void main(String[] args){
int count = 1;
while (count < 11) {
System.out.println("Count is: " + count);
count+
}
}
}
You can implement an infinite loop using the while statement as follows:
while (true){
// your code goes here
}
The Java programming language also provides a do-while statement, which can be expressed as follows:
do {
statement(s)
} while (expression);
The difference between do-while and while is that do-while evaluates its expression at the bottom of the loop instead of the top. Therefore, the statements within the do block are always executed at least once, as shown in the following DoWhileDemo program:

class DoWhileDemo {
public static void main(String[] args){
int count = 1;
do {
System.out.println("Count is: " + count);
count+
} while (count <= 11);

The for Statement
The for statement provides a compact way to iterate over a range of values. Programmers often refer to it as the "for loop" because of the way in which it repeatedly loops until a particular condition is satisfied. The general form of the for statement can be expressed as follows:
for (initialization; termination; increment) {
statement(s)
}
When using this version of the for statement, keep in mind that:
• The initialization expression initializes the loop; it's executed once, as the loop begins.
• When the termination expression evaluates to false, the loop terminates.
• The increment expression is invoked after each iteration through the loop; it is perfectly acceptable for this expression to increment or decrement a value.
The following program, ForDemo, uses the general form of the for statement to print the numbers 1 through 10 to standard output:

class ForDemo {
public static void main(String[] args){
for(int i=1; i<11; i+{
System.out.println("Count is: " + i);
}
}
}
The output of this program is:
Count is: 1
Count is: 2
Count is: 3
Count is: 4
Count is: 5
Count is: 6
Count is: 7
Count is: 8
Count is: 9
Count is: 10
Notice how the code declares a variable within the initialization expression. The scope of this variable extends from its declaration to the end of the block governed by the for statement, so it can be used in the termination and increment expressions as well. If the variable that controls a for statement is not needed outside of the loop, it's best to declare the variable in the initialization expression. The names i, j, and k are often used to control for loops; declaring them within the initialization expression limits their life span and reduces errors.
The three expressions of the for loop are optional; an infinite loop can be created as follows:
for ( ; ; ) { // infinite loop

// your code goes here
}
The for statement also has another form designed for iteration through Collections and arrays This form is sometimes referred to as the enhanced for statement, and can be used to make your loops more compact and easy to read. To demonstrate, consider the following array, which holds the numbers 1 through 10:
int[] numbers = {1,2,3,4,5,6,7,8,9,1 0};
The following program, EnhancedForDemo, uses the enhanced for to loop through the array:

class EnhancedForDemo {
public static void main(String[] args){
int[] numbers = {1,2,3,4,5,6,7,8,9,1 0};
for (int item : numbers) {
System.out.println("Count is: " + item);
}
}
}
In this example, the variable item holds the current value from the numbers array. The output from this program is the same as before:
Count is: 1
Count is: 2
Count is: 3
Count is: 4
Count is: 5
Count is: 6
Count is: 7
Count is: 8
Count is: 9
Count is: 10
We recommend using this form of the for statement instead of the general form whenever possible.


Branching Statements
The break Statement
The break statement has two forms: labeled and unlabeled. You saw the unlabeled form in the previous discussion of the switch statement. You can also use an unlabeled break to terminate a for, while, or do-while loop, as shown in the following BreakDemo program:
class BreakDemo {
public static void main(String[] args) {

int[] arrayOfInts = { 32, 87, 3, 589, 12, 1076,
2000, 8, 622, 127 };
int searchfor = 12;

int i;
boolean foundIt = false;

for (i = 0; i < arrayOfInts.length; i+ {
if (arrayOfInts[i] == searchfor) {
foundIt = true;
break;
}
}

if (foundIt) {
System.out.println("Found " + searchfor
+ " at index " + i);
} else {
System.out.println(s earchfor
+ " not in the array");
}
}
}
This program searches for the number 12 in an array. The break statement, shown in boldface, terminates the for loop when that value is found. Control flow then transfers to the print statement at the end of the program. This program's output is:
Found 12 at index 4
An unlabeled break statement terminates the innermost switch, for, while, or do-while statement, but a labeled break terminates an outer statement. The following program, BreakWithLabelDemo, is similar to the previous program, but uses nested for loops to search for a value in a two-dimensional array. When the value is found, a labeled break terminates the outer for loop (labeled "search"):

class BreakWithLabelDemo {
public static void main(String[] args) {

int[][] arrayOfInts = { { 32, 87, 3, 589 },
{ 12, 1076, 2000, 8 },
{ 622, 127, 77, 955 }
};
int searchfor = 12;

int i;
int j = 0;
boolean foundIt = false;

search:
for (i = 0; i < arrayOfInts.length; i+ {
for (j = 0; j < arrayOfInts[i].length; j+ {
if (arrayOfInts[i][j] == searchfor) {
foundIt = true;
break search;
}
}
}

if (foundIt) {
System.out.println("Found " + searchfor +
" at " + i + ", " + j);
} else {
System.out.println(s earchfor
+ " not in the array");
}
}
}
This is the output of the program.
Found 12 at 1, 0
The break statement terminates the labeled statement; it does not transfer the flow of control to the label. Control flow is transferred to the statement immediately following the labeled (terminated) statement.
The continue Statement
The continue statement skips the current iteration of a for, while , or do-while loop. The unlabeled form skips to the end of the innermost loop's body and evaluates the boolean expression that controls the loop. The following program, ContinueDemo , steps through a String, counting the occurences of the letter "p". If the current character is not a p, the continue statement skips the rest of the loop and proceeds to the next character. If it is a "p", the program increments the letter count.

class ContinueDemo {
public static void main(String[] args) {

String searchMe = "peter piper picked a peck of pickled peppers";
int max = searchMe.length();
int numPs = 0;

for (int i = 0; i < max; i+ {
//interested only in p's
if (searchMe.charAt(i) != 'p')
continue;

//process p's
numPs+
}
System.out.println("Found " + numPs + " p's in the string.");
}
}
Here is the output of this program:
Found 9 p's in the string.
To see this effect more clearly, try removing the continue statement and recompiling. When you run the program again, the count will be wrong, saying that it found 35 p's instead of 9.
A labeled continue statement skips the current iteration of an outer loop marked with the given label. The following example program, ContinueWithLabelDem o, uses nested loops to search for a substring within another string. Two nested loops are required: one to iterate over the substring and one to iterate over the string being searched. The following program, ContinueWithLabelDem o, uses the labeled form of continue to skip an iteration in the outer loop.

class ContinueWithLabelDem o {
public static void main(String[] args) {

String searchMe = "Look for a substring in me";
String substring = "sub";
boolean foundIt = false;

int max = searchMe.length() - substring.length();

test:
for (int i = 0; i <= max; i+ {
int n = substring.length();
int j = i;
int k = 0;
while (n-- != 0) {
if (searchMe.charAt(j++ )
!= substring.charAt(k++ )) {
continue test;
}
}
foundIt = true;
break test;
}
System.out.println(f oundIt ? "Found it" :
"Didn't find it");
}
}
Here is the output from this program.
Found it


The return Statement
The last of the branching statements is the return statement. The return statement exits from the current method, and control flow returns to where the method was invoked. The return statement has two forms: one that returns a value, and one that doesn't. To return a value, simply put the value (or an expression that calculates the value) after the return keyword.
return ++count;
The data type of the returned value must match the type of the method's declared return value. When a method is declared void, use the form of return that doesn't return a value.
return;

Lesson: Classes and Objects
With the knowledge you now have of the basics of the Java programming language, you can learn to write your own classes. In this lesson, you will find information about defining your own classes, including declaring member variables, methods, and constructors.
You will learn to use your classes to create objects, and how to use the objects you create.
This lesson also covers nesting classes within other classes, enumerations, and annotations.
Classes
This section shows you the anatomy of a class, and how to declare fields, methods, and constructors.
Objects
This section covers creating and using objects. You will learn how to instantiate an object, and, once instantiated, how to use the dot operator to access the object's instance variables and methods.
More on Classes
This section covers more aspects of classes that depend on using object references and the dot operator that you learned about in the preceding section: returning values from methods, the this keyword, class vs. instance members, and access control.
Nested Classes
Static nested classes, inner classes, anonymous inner classes, and local classes are covered.
Enum Types
This section covers enumerations, specialized classes that allow you to define and use sets of constants.
Annotations
Annotations allow you to add information to your program that is not actually part of the program. This section describes three built-in annotations that you should know about.

Classes
The introduction to object-oriented concepts in the lesson titled Object-oriented Programming Concepts used a bicycle class as an example, with racing bikes, mountain bikes, and tandem bikes as subclasses. Here is sample code for a possible implementation of a Bicycle class, to give you an overview of a class declaration. Subsequent sections of this lesson will back up and explain class declarations step by step. For the moment, don't concern yourself with the details.
public class Bicycle {

// the Bicycle class has three fields
public int cadence;
public int gear;
public int speed;

// the Bicycle class has one constructor
public Bicycle(int startCadence, int startSpeed, int startGear) {
gear = startGear;
cadence = startCadence;
speed = startSpeed;
}

// the Bicycle class has four methods
public void setCadence(int newValue) {
cadence = newValue;
}

public void setGear(int newValue) {
gear = newValue;
}

public void applyBrake(int decrement) {
speed -= decrement;
}

public void speedUp(int increment) {
speed += increment;
}

}
A class declaration for a MountainBike class that is a subclass of Bicycle might look like this:
public class MountainBike extends Bicycle {

// the MountainBike subclass has one field
public int seatHeight;

// the MountainBike subclass has one constructor
public MountainBike(int startHeight, int startCadence, int startSpeed, int startGear) {
super(startCadence, startSpeed, startGear);
seatHeight = startHeight;
}

// the MountainBike subclass has one method
public void setHeight(int newValue) {
seatHeight = newValue;
}

}
MountainBike inherits all the fields and methods of Bicycle and adds the field seatHeight and a method to set it (mountain bikes have seats that can be moved up and down as the terrain demands).

Declaring Classes
You've seen classes defined in the following way:
class MyClass {
//field, constructor, and method declarations
}
This is a class declaration. The class body (the area between the braces) contains all the code that provides for the life cycle of the objects created from the class: constructors for initializing new objects, declarations for the fields that provide the state of the class and its objects, and methods to implement the behavior of the class and its objects.
The preceding class declaration is a minimal one—it contains only those components of a class declaration that are required. You can provide more information about the class, such as the name of its superclass, whether it implements any interfaces, and so on, at the start of the class declaration. For example,
class MyClass extends MySuperClass implements YourInterface {
//field, constructor, and method declarations
}
means that MyClass is a subclass of MySuperClass and that it implements the YourInterface interface.
You can also add modifiers like public or private at the very beginning—so you can see that the opening line of a class declaration can become quite complicated. The modifiers public and private, which determine what other classes can access MyClass, are discussed later in this lesson. The lesson on interfaces and inheritance will explain how and why you would use the extends and implements keywords in a class declaration. For the moment you do not need to worry about these extra complications.
In general, class declarations can include these components, in order:
1. Modifiers such as public, private, and a number of others that you will encounter later.
2. The class name, with the initial letter capitalized by convention.
3. The name of the class's parent (superclass), if any, preceded by the keyword extends. A class can only extend (subclass) one parent.
4. A comma-separated list of interfaces implemented by the class, if any, preceded by the keyword implements. A class can implement more than one interface.
5. The class body, surrounded by braces, {}.
Declaring Member Variables
There are several kinds of variables:
• Member variables in a class—these are called fields.
• Variables in a method or block of code—these are called local variables.
• Variables in method declarations—these are called parameters.
The Bicycle class uses the following lines of code to define its fields:
public int cadence;
public int gear;
public int speed;
Field declarations are composed of three components, in order:
1. Zero or more modifiers, such as public or private.
2. The field's type.
3. The field's name.
The fields of Bicycle are named cadence, gear, and speed and are all of data type integer (int). The public keyword identifies these fields as public members, accessible by any object that can access the class.
Access Modifiers
The first (left-most) modifier used lets you control what other classes have access to a member field. For the moment, consider only public and private. Other access modifiers will be discussed later.
• public modifier—the field is accessible from all classes.
• private modifier—the field is accessible only within its own class.
In the spirit of encapsulation, it is common to make fields private. This means that they can only be directly accessed from the Bicycle class. We still need access to these values, however. This can be done indirectly by adding public methods that obtain the field values for us:
public class Bicycle {

private int cadence;
private int gear;
private int speed;

public Bicycle(int startCadence, int startSpeed, int startGear) {
gear = startGear;
cadence = startCadence;
speed = startSpeed;
}

public int getCadence() {
return cadence;
}

public void setCadence(int newValue) {
cadence = newValue;
}

public int getGear() {
return gear;
}

public void setGear(int newValue) {
gear = newValue;
}

public int getSpeed() {
return speed;
}

public void applyBrake(int decrement) {
speed -= decrement;
}

public void speedUp(int increment) {
speed += increment;
}

}
Types
All variables must have a type. You can use primitive types such as int, float, boolean, etc. Or you can use reference types, such as strings, arrays, or objects.
Variable Names
All variables, whether they are fields, local variables, or parameters, follow the same naming rules and conventions that were covered in the Language Basics lesson, Variables—Naming .
In this lesson, be aware that the same naming rules and conventions are used for method and class names, except that
• the first letter of a class name should be capitalized, and
• the first (or only) word in a method name should be a verb.

Defining Methods
Here is an example of a typical method declaration:
public double calculateAnswer(doub le wingSpan, int numberOfEngines, double length, double grossTons) {
//do the calculation here
}
The only required elements of a method declaration are the method's return type, name, a pair of parentheses, (), and a body between braces, {}.
More generally, method declarations have six components, in order:
1. Modifiers—such as public, private, and others you will learn about later.
2. The return type—the data type of the value returned by the method, or void if the method does not return a value.
3. The method name—the rules for field names apply to method names as well, but the convention is a little different.
4. The parameter list in parenthesis—a comma-delimited list of input parameters, preceded by their data types, enclosed by parentheses, (). If there are no parameters, you must use empty parentheses.
5. An exception list—to be discussed later.
6. The method body, enclosed between braces—the method's code, including the declaration of local variables, goes here.
Modifiers, return types, and parameters will be discussed later in this lesson. Exceptions are discussed in a later lesson.
____________________ ____________________
Definition: Two of the components of a method declaration comprise the method signature—the method's name and the parameter types.
____________________ ____________________
The signature of the method declared above is:
calculateAnswer(doub le, int, double, double)
Naming a Method
Although a method name can be any legal identifier, code conventions restrict method names. By convention, method names should be a verb in lowercase or a multi-word name that begins with a verb in lowercase, followed by adjectives, nouns, etc. In multi-word names, the first letter of each of the second and following words should be capitalized. Here are some examples:
run
runFast
getBackground
getFinalData
compareTo
setX
isEmpty
Typically, a method has a unique name within its class. However, a method might have the same name as other methods due to method overloading.
Overloading Methods
The Java programming language supports overloading methods, and Java can distinguish between methods with different method signatures. This means that methods within a class can have the same name if they have different parameter lists (there are some qualifications to this that will be discussed in the lesson titled "Interfaces and Inheritance").
Suppose that you have a class that can use calligraphy to draw various types of data (strings, integers, and so on) and that contains a method for drawing each data type. It is cumbersome to use a new name for each method—for example, drawString, drawInteger, drawFloat, and so on. In the Java programming language, you can use the same name for all the drawing methods but pass a different argument list to each method. Thus, the data drawing class might declare four methods named draw, each of which has a different parameter list.
public class DataArtist {
...
public void draw(String s) {
...
}
public void draw(int i) {
...
}
public void draw(double f) {
...
}
public void draw(int i, double f) {
...
}
}
Overloaded methods are differentiated by the number and the type of the arguments passed into the method. In the code sample, draw(String s) and draw(int i) are distinct and unique methods because they require different argument types.
You cannot declare more than one method with the same name and the same number and type of arguments, because the compiler cannot tell them apart.
The compiler does not consider return type when differentiating methods, so you cannot declare two methods with the same signature even if they have a different return type.
____________________ ____________________
Note: Overloaded methods should be used sparingly, as they can make code much less readable.
Providing Constructors for Your Classes
A class contains constructors that are invoked to create objects from the class blueprint. Constructor declarations look like method declarations—excep t that they use the name of the class and have no return type. For example, Bicycle has one constructor:
public Bicycle(int startCadence, int startSpeed, int startGear) {
gear = startGear;
cadence = startCadence;
speed = startSpeed;
}
To create a new Bicycle object called myBike, a constructor is called by the new operator:
Bicycle myBike = new Bicycle(30, 0, 8);
new Bicycle(30, 0, 8) creates space in memory for the object and initializes its fields.
Although Bicycle only has one constructor, it could have others, including a no-argument constructor:
public Bicycle() {
gear = 1;
cadence = 10;
speed = 0;
}
Bicycle yourBike = new Bicycle(); invokes the no-argument constructor to create a new Bicycle object called yourBike.
Both constructors could have been declared in Bicycle because they have different argument lists. As with methods, the Java platform differentiates constructors on the basis of the number of arguments in the list and their types. You cannot write two constructors that have the same number and type of arguments for the same class, because the platform would not be able to tell them apart. Doing so causes a compile-time error.
You don't have to provide any constructors for your class, but you must be careful when doing this. The compiler automatically provides a no-argument, default constructor for any class without constructors. This default constructor will call the no-argument constructor of the superclass. In this situation, the compiler will complain if the superclass doesn't have a no-argument constructor so you must verify that it does. If your class has no explicit superclass, then it has an implicit superclass of Object, which does have a no-argument constructor.
You can use a superclass constructor yourself. The MountainBike class at the beginning of this lesson did just that. This will be discussed later, in the lesson on interfaces and inheritance.
You can use access modifiers in a constructor's declaration to control which other classes can call the constructor.
____________________ ____________________
Note : If another class cannot call a MyClass constructor, it cannot directly create MyClass objects.

Passing Information to a Method or a Constructor
The declaration for a method or a constructor declares the number and the type of the arguments for that method or constructor. For example, the following is a method that computes the monthly payments for a home loan, based on the amount of the loan, the interest rate, the length of the loan (the number of periods), and the future value of the loan:
public double computePayment(doubl e loanAmt,
double rate,
double futureValue,
int numPeriods) {
double interest = rate / 100.0;
double partial1 = Math.pow((1 + interest), -numPeriods);
double denominator = (1 - partial1) / interest;
double answer = (-loanAmt / denominator)
- ((futureValue * partial1) / denominator);
return answer;
}
This method has four parameters: the loan amount, the interest rate, the future value and the number of periods. The first three are double-precision floating point numbers, and the fourth is an integer. The parameters are used in the method body and at runtime will take on the values of the arguments that are passed in.
____________________ ____________________
Note : Parameters refers to the list of variables in a method declaration. Arguments are the actual values that are passed in when the method is invoked. When you invoke a method, the arguments used must match the declaration's parameters in type and order.
____________________ ____________________
Parameter Types
You can use any data type for a parameter of a method or a constructor. This includes primitive data types, such as doubles, floats, and integers, as you saw in the computePayment method, and reference data types, such as objects and arrays.
Here's an example of a method that accepts an array as an argument. In this example, the method creates a new Polygon object and initializes it from an array of Point objects (assume that Point is a class that represents an x, y coordinate):
public Polygon polygonFrom(Point[] corners) {
// method body goes here
}
____________________ ____________________
Note : The Java programming language doesn't let you pass methods into methods. But you can pass an object into a method and then invoke the object's methods.
____________________ ____________________
Arbitrary Number of Arguments
You can use a construct called varargs to pass an arbitrary number of values to a method. You use varargs when you don't know how many of a particular type of argument will be passed to the method. It's a shortcut to creating an array manually (the previous method could have used varargs rather than an array).
To use varargs, you follow the type of the last parameter by an ellipsis (three dots, ...), then a space, and the parameter name. The method can then be called with any number of that parameter, including none.
public Polygon polygonFrom(Point... corners) {
int numberOfSides = corners.length;
double squareOfSide1, lengthOfSide1;
squareOfSide1 = (corners[1].x - corners[0].x)*(corners[1].x - corners[0].x)
+ (corners[1].y - corners[0].y)*(corners[1].y - corners[0].y) ;
lengthOfSide1 = Math.sqrt(squareOfSi de1);
// more method body code follows that creates
// and returns a polygon connecting the Points
}
You can see that, inside the method, corners is treated like an array. The method can be called either with an array or with a sequence of arguments. The code in the method body will treat the parameter as an array in either case.
You will most commonly see varargs with the printing methods; for example, this printf method:
public PrintStream printf(String format, Object... args)
allows you to print an arbitrary number of objects. It can be called like this:
System.out.printf("%s: %d, %s%n", name, idnum, address);
or like this
System.out.printf("%s: %d, %s, %s, %s%n", name, idnum, address, phone, email);
or with yet a different number of arguments.
Parameter Names
When you declare a parameter to a method or a constructor, you provide a name for that parameter. This name is used within the method body to refer to the passed-in argument.
The name of a parameter must be unique in its scope. It cannot be the same as the name of another parameter for the same method or constructor, and it cannot be the name of a local variable within the method or constructor.
A parameter can have the same name as one of the class's fields. If this is the case, the parameter is said to shadow the field. Shadowing fields can make your code difficult to read and is conventionally used only within constructors and methods that set a particular field. For example, consider the following Circle class and its setOrigin method:
public class Circle {
private int x, y, radius;
public void setOrigin(int x, int y) {
...
}
}
The Circle class has three fields: x, y, and radius. The setOrigin method has two parameters, each of which has the same name as one of the fields. Each method parameter shadows the field that shares its name. So using the simple names x or y within the body of the method refers to the parameter, not to the field. To access the field, you must use a qualified name. This will be discussed later in this lesson in the section titled "Using the this Keyword."
Passing Primitive Data Type Arguments
Primitive arguments, such as an int or a double, are passed into methods by value. This means that any changes to the values of the parameters exist only within the scope of the method. When the method returns, the parameters are gone and any changes to them are lost. Here is an example:
public class PassPrimitiveByValue {

public static void main(String[] args) {

int x = 3;

//invoke passMethod() with x as argument
passMethod(x);

// print x to see if its value has changed
System.out.println("After invoking passMethod, x = " + x);

}

// change parameter in passMethod()
public static void passMethod(int p) {
p = 10;
}
}
When you run this program, the output is:
After invoking passMethod, x = 3
Passing Reference Data Type Arguments
Reference data type parameters, such as objects, are also passed into methods by value. This means that when the method returns, the passed-in reference still references the same object as before. However, the values of the object's fields can be changed in the method, if they have the proper access level.
For example, consider a method in an arbitrary class that moves Circle objects:
public void moveCircle(Circle circle, int deltaX, int deltaY) {
// code to move origin of circle to x+deltaX, y+deltaY
circle.setX(circle.g etX() + deltaX);
circle.setY(circle.g etY() + deltaY);

//code to assign a new reference to circle
circle = new Circle(0, 0);
}
Let the method be invoked with these arguments:
moveCircle(myCircle, 23, 56)
Inside the method, circle initially refers to myCircle. The method changes the x and y coordinates of the object that circle references (i.e., myCircle) by 23 and 56, respectively. These changes will persist when the method returns. Then circle is assigned a reference to a new Circle object with x = y = 0. This reassignment has no permanence, however, because the reference was passed in by value and cannot change. Within the method, the object pointed to by circle has changed, but, when the method returns, myCircle still references the same Circle object as before the method was called.

Objects
A typical Java program creates many objects, which as you know, interact by invoking methods. Through these object interactions, a program can carry out various tasks, such as implementing a GUI, running an animation, or sending and receiving information over a network. Once an object has completed the work for which it was created, its resources are recycled for use by other objects.
Here's a small program, called CreateObjectDemo, that creates three objects: one Point object and two Rectangle objects. You will need all three source files to compile this program.

public class CreateObjectDemo {

public static void main(String[] args) {

//Declare and create a point object
//and two rectangle objects.
Point originOne = new Point(23, 94);
Rectangle rectOne = new Rectangle(originOne, 100, 200);
Rectangle rectTwo = new Rectangle(50, 100);

//display rectOne's width, height, and area
System.out.println("Width of rectOne: " +
rectOne.width);
System.out.println("Height of rectOne: " +
rectOne.height);
System.out.println("Area of rectOne: " + rectOne.getArea());

//set rectTwo's position
rectTwo.origin = originOne;

//display rectTwo's position
System.out.println("X Position of rectTwo: "
+ rectTwo.origin.x);
System.out.println("Y Position of rectTwo: "
+ rectTwo.origin.y);

//move rectTwo and display its new position
rectTwo.move(40, 72);
System.out.println("X Position of rectTwo: "
+ rectTwo.origin.x);
System.out.println("Y Position of rectTwo: "
+ rectTwo.origin.y);
}
}
This program creates, manipulates, and displays information about various objects. Here's the output:
Width of rectOne: 100
Height of rectOne: 200
Area of rectOne: 20000
X Position of rectTwo: 23
Y Position of rectTwo: 94
X Position of rectTwo: 40
Y Position of rectTwo: 72
The following three sections use the above example to describe the life cycle of an object within a program. From them, you will learn how to write code that creates and uses objects in your own programs. You will also learn how the system cleans up after an object when its life has ended.

Creating Objects
As you know, a class provides the blueprint for objects; you create an object from a class. Each of the following statements taken from the CreateObjectDemo program creates an object and assigns it to a variable:
Point originOne = new Point(23, 94);
Rectangle rectOne = new Rectangle(originOne, 100, 200);
Rectangle rectTwo = new Rectangle(50, 100);
The first line creates an object of the Point class, and the second and third lines each create an object of the Rectangle class.
Each of these statements has three parts (discussed in detail below):
1. Declaration: The code set in bold are all variable declarations that associate a variable name with an object type.
2. Instantiation: The new keyword is a Java operator that creates the object.
3. Initialization: The new operator is followed by a call to a constructor, which initializes the new object.
Declaring a Variable to Refer to an Object
Previously, you learned that to declare a variable, you write:
type name;
This notifies the compiler that you will use name to refer to data whose type is type. With a primitive variable, this declaration also reserves the proper amount of memory for the variable.
You can also declare a reference variable on its own line. For example:
Point originOne;
If you declare originOne like this, its value will be undetermined until an object is actually created and assigned to it. Simply declaring a reference variable does not create an object. For that, you need to use the new operator, as described in the next section. You must assign an object to originOne before you use it in your code. Otherwise, you will get a compiler error.
A variable in this state, which currently references no object, can be illustrated as follows (the variable name, originOne, plus a reference pointing to nothing):

Instantiating a Class
The new operator instantiates a class by allocating memory for a new object and returning a reference to that memory. The new operator also invokes the object constructor.
____________________ ____________________
Note: The phrase "instantiating a class" means the same thing as "creating an object." When you create an object, you are creating an "instance" of a class, therefore "instantiating" a class.
____________________ ____________________
The new operator requires a single, postfix argument: a call to a constructor. The name of the constructor provides the name of the class to instantiate.
The new operator returns a reference to the object it created. This reference is usually assigned to a variable of the appropriate type, like:
Point originOne = new Point(23, 94);
The reference returned by the new operator does not have to be assigned to a variable. It can also be used directly in an expression. For example:
int height = new Rectangle().height;
This statement will be discussed in the next section.
Initializing an Object
Here's the code for the Point class:
public class Point {
public int x = 0;
public int y = 0;
//constructor
public Point(int a, int b) {
x = a;
y = b;
}
}
This class contains a single constructor. You can recognize a constructor because its declaration uses the same name as the class and it has no return type. The constructor in the Point class takes two integer arguments, as declared by the code (int a, int b). The following statement provides 23 and 94 as values for those arguments:
Point originOne = new Point(23, 94);
The result of executing this statement can be illustrated in the next figure:

Here's the code for the Rectangle class, which contains four constructors:
public class Rectangle {
public int width = 0;
public int height = 0;
public Point origin;

// four constructors
public Rectangle() {
origin = new Point(0, 0);
}
public Rectangle(Point p) {
origin = p;
}
public Rectangle(int w, int h) {
origin = new Point(0, 0);
width = w;
height = h;
}
public Rectangle(Point p, int w, int h) {
origin = p;
width = w;
height = h;
}

// a method for moving the rectangle
public void move(int x, int y) {
origin.x = x;
origin.y = y;
}

// a method for computing the area of the rectangle
public int getArea() {
return width * height;
}
}

Each constructor lets you provide initial values for the rectangle's size and width, using both primitive and reference types. If a class has multiple constructors, they must have different signatures. The Java compiler differentiates the constructors based on the number and the type of the arguments. When the Java compiler encounters the following code, it knows to call the constructor in the Rectangle class that requires a Point argument followed by two integer arguments:

Rectangle rectOne = new Rectangle(originOne, 100, 200);
This calls one of Rectangle's constructors that initializes origin to originOne. Also, the constructor sets width to 100 and height to 200. Now there are two references to the same Point object— an object can have multiple references to it, as shown in the next figure:

The following line of code calls the Rectangle constructor that requires two integer arguments, which provide the initial values for width and height. If you inspect the code within the constructor, you will see that it creates a new Point object whose x and y values are initialized to 0:
Rectangle rectTwo = new Rectangle(50, 100);
The Rectangle constructor used in the following statement doesn't take any arguments, so it's called a no-argument constructor:
Rectangle rect = new Rectangle();
All classes have at least one constructor. If a class does not explicitly declare any, the Java compiler automatically provides a no-argument constructor, called the default constructor. This default constructor calls the class parent's no-argument constructor, or the Object constructor if the class has no other parent. If the parent has no constructor (Object does have one), the compiler will reject the program.


Using Objects
Once you've created an object, you probably want to use it for something. You may need to use the value of one of its fields, change one of its fields, or call one of its methods to perform an action.
Referencing an Object's Fields
Object fields are accessed by their name. You must use a name that is unambiguous.
You may use a simple name for a field within its own class. For example, we can add a statement within the Rectangle class that prints the width and height:
System.out.println("Width and height are: " + width + ", " + height);
In this case, width and height are simple names.
Code that is outside the object's class must use an object reference or expression, followed by the dot (.) operator, followed by a simple field name, as in:
objectReference.fiel dName
For example, the code in the CreateObjectDemo class is outside the code for the Rectangle class. So to refer to the origin, width, and height fields within the Rectangle object named rectOne, the CreateObjectDemo class must use the names rectOne.origin, rectOne.width, and rectOne.height, respectively. The program uses two of these names to display the width and the height of rectOne:
System.out.println("Width of rectOne: " + rectOne.width);
System.out.println("Height of rectOne: " + rectOne.height);
Attempting to use the simple names width and height from the code in the CreateObjectDemo class doesn't make sense — those fields exist only within an object — and results in a compiler error.
Later, the program uses similar code to display information about rectTwo. Objects of the same type have their own copy of the same instance fields. Thus, each Rectangle object has fields named origin, width, and height. When you access an instance field through an object reference, you reference that particular object's field. The two objects rectOne and rectTwo in the CreateObjectDemo program have different origin, width, and height fields.
To access a field, you can use a named reference to an object, as in the previous examples, or you can use any expression that returns an object reference. Recall that the new operator returns a reference to an object. So you could use the value returned from new to access a new object's fields:
int height = new Rectangle().height;
This statement creates a new Rectangle object and immediately gets its height. In essence, the statement calculates the default height of a Rectangle. Note that after this statement has been executed, the program no longer has a reference to the created Rectangle, because the program never stored the reference anywhere. The object is unreferenced, and its resources are free to be recycled by the Java Virtual Machine.
Calling an Object's Methods
You also use an object reference to invoke an object's method. You append the method's simple name to the object reference, with an intervening dot operator (.). Also, you provide, within enclosing parentheses, any arguments to the method. If the method does not require any arguments, use empty parentheses.
objectReference.meth odName(argumentList) ;
or
objectReference.meth odName();
The Rectangle class has two methods: getArea() to compute the rectangle's area and move() to change the rectangle's origin. Here's the CreateObjectDemo code that invokes these two methods:
System.out.println("Area of rectOne: " + rectOne.getArea());
...
rectTwo.move(40, 72);
The first statement invokes rectOne's getArea() method and displays the results. The second line moves rectTwo because the move() method assigns new values to the object's origin.x and origin.y.
As with instance fields, objectReference must be a reference to an object. You can use a variable name, but you also can use any expression that returns an object reference. The new operator returns an object reference, so you can use the value returned from new to invoke a new object's methods:
new Rectangle(100, 50).getArea()
The expression new Rectangle(100, 50) returns an object reference that refers to a Rectangle object. As shown, you can use the dot notation to invoke the new Rectangle's getArea() method to compute the area of the new rectangle.
Some methods, such as getArea(), return a value. For methods that return a value, you can use the method invocation in expressions. You can assign the return value to a variable, use it to make decisions, or control a loop. This code assigns the value returned by getArea() to the variable areaOfRectangle:
int areaOfRectangle = new Rectangle(100, 50).getArea();
Remember, invoking a method on a particular object is the same as sending a message to that object. In this case, the object that getArea() is invoked on is the rectangle returned by the constructor.
The Garbage Collector
Some object-oriented languages require that you keep track of all the objects you create and that you explicitly destroy them when they are no longer needed. Managing memory explicitly is tedious and error-prone. The Java platform allows you to create as many objects as you want (limited, of course, by what your system can handle), and you don't have to worry about destroying them. The Java runtime environment deletes objects when it determines that they are no longer being used. This process is called garbage collection.
An object is eligible for garbage collection when there are no more references to that object. References that are held in a variable are usually dropped when the variable goes out of scope. Or, you can explicitly drop an object reference by setting the variable to the special value null. Remember that a program can have multiple references to the same object; all references to an object must be dropped before the object is eligible for garbage collection.
The Java runtime environment has a garbage collector that periodically frees the memory used by objects that are no longer referenced. The garbage collector does its job automatically when it determines that the time is right.

More on Classes
This section covers more aspects of classes that depend on using object references and the dot operator that you learned about in the preceding sections on objects:
• Returning values from methods.
• The this keyword.
• Class vs. instance members.
• Access control

Returning a Value from a Method
A method returns to the code that invoked it when it
• completes all the statements in the method,
• reaches a return statement, or
• throws an exception (covered later),
whichever occurs first.
You declare a method's return type in its method declaration. Within the body of the method, you use the return statement to return the value.
Any method declared void doesn't return a value. It does not need to contain a return statement, but it may do so. In such a case, a return statement can be used to branch out of a control flow block and exit the method and is simply used like this:
return;

If you try to return a value from a method that is declared void, you will get a compiler error.
Any method that is not declared void must contain a return statement with a corresponding return value, like this:
return returnValue;

The data type of the return value must match the method's declared return type; you can't return an integer value from a method declared to return a boolean.
The getArea() method in the Rectangle Rectangle class that was discussed in the sections on objects returns an integer:
// a method for computing the area of the rectangle
public int getArea() {
return width * height;
}
This method returns the integer that the expression width*height evaluates to.
The area method returns a primitive type. A method can also return a reference type. For example, in a program to manipulate Bicycle objects, we might have a method like this:
public Bicycle seeWhosFastest(Bicyc le myBike, Bicycle yourBike, Environment env) {
Bicycle fastest;
// code to calculate which bike is faster, given
// each bike's gear and cadence and given
// the environment (terrain and wind)
return fastest;
}
Returning a Class or Interface
If this section confuses you, skip it and return to it after you have finished the lesson on interfaces and inheritance.
When a method uses a class name as its return type, such as whosFastest does, the class of the type of the returned object must be either a subclass of, or the exact class of, the return type. Suppose that you have a class hierarchy in which ImaginaryNumber is a subclass of java.lang.Number, which is in turn a subclass of Object, as illustrated in the following figure.

The class hierarchy for ImaginaryNumber
Now suppose that you have a method declared to return a Number:
public Number returnANumber() {
...
}
The returnANumber method can return an ImaginaryNumber but not an Object. ImaginaryNumber is a Number because it's a subclass of Number. However, an Object is not necessarily a Number — it could be a String or another type.
You can override a method and define it to return a subclass of the original method, like this:
public ImaginaryNumber returnANumber() {
...
}
This technique, called covariant return type, means that the return type is allowed to vary in the same direction as the subclass.
____________________ ____________________
Note: You also can use interface names as return types. In this case, the object returned must implement the specified interface.


Using the this Keyword
Within an instance method or a constructor, this is a reference to the current object — the object whose method or constructor is being called. You can refer to any member of the current object from within an instance method or a constructor by using this.
Using this with a Field
The most common reason for using the this keyword is because a field is shadowed by a method or constructor parameter.
For example, the Point class was written like this
public class Point {
public int x = 0;
public int y = 0;

//constructor
public Point(int a, int b) {
x = a;
y = b;
}
}
but it could have been written like this:
public class Point {
public int x = 0;
public int y = 0;

//constructor
public Point(int x, int y) {
this.x = x;
this.y = y;
}
}
Each argument to the second constructor shadows one of the object's fields—inside the constructor x is a local copy of the constructor's first argument. To refer to the Point field x, the constructor must use this.x.
Using this with a Constructor
From within a constructor, you can also use the this keyword to call another constructor in the same class. Doing so is called an explicit constructor invocation. Here's another Rectangle class, with a different implementation from the one in the Objects section.
public class Rectangle {
private int x, y;
private int width, height;

public Rectangle() {
this(0, 0, 0, 0);
}
public Rectangle(int width, int height) {
this(0, 0, width, height);
}
public Rectangle(int x, int y, int width, int height) {
this.x = x;
this.y = y;
this.width = width;
this.height = height;
}
...
}
This class contains a set of constructors. Each constructor initializes some or all of the rectangle's member variables. The constructors provide a default value for any member variable whose initial value is not provided by an argument. For example, the no-argument constructor calls the four-argument constructor with four 0 values and the two-argument constructor calls the four-argument constructor with two 0 values. As before, the compiler determines which constructor to call, based on the number and the type of arguments.
If present, the invocation of another constructor must be the first line in the constructor.


Controlling Access to Members of a Class
Access level modifiers determine whether other classes can use a particular field or invoke a particular method. There are two levels of access control:
• At the top level—public, or package-private (no explicit modifier).
• At the member level—public, private, protected, or package-private (no explicit modifier).
A class may be declared with the modifier public, in which case that class is visible to all classes everywhere. If a class has no modifier (the default, also known as package-private), it is visible only within its own package (packages are named groups of related classes—you will learn about them in a later lesson.)
At the member level, you can also use the public modifier or no modifier (package-private) just as with top-level classes, and with the same meaning. For members, there are two additional access modifiers: private and protected. The private modifier specifies that the member can only be accessed in its own class. The protected modifier specifies that the member can only be accessed within its own package (as with package-private) and, in addition, by a subclass of its class in another package.
The following table shows the access to members permitted by each modifier.
Access Levels
Modifier Class Package Subclass World
public Y Y Y Y
protected Y Y Y N
no modifier Y Y N N
private Y N N N
The first data column indicates whether the class itself has access to the member defined by the access level. As you can see, a class always has access to its own members. The second column indicates whether classes in the same package as the class (regardless of their parentage) have access to the member. The third column indicates whether subclasses of the class — declared outside this package — have access to the member. The fourth column indicates whether all classes have access to the member.
Access levels affect you in two ways. First, when you use classes that come from another source, such as the classes in the Java platform, access levels determine which members of those classes your own classes can use. Second, when you write a class, you need to decide what access level every member variable and every method in your class should have.
Let's look at a collection of classes and see how access levels affect visibility. The following figure shows the four classes in this example and how they are related.

Classes and Packages of the Example Used to Illustrate Access Levels
The following table shows where the members of the Alpha class are visible for each of the access modifiers that can be applied to them.
Visibility
Modifier Alpha Beta Alphasub Gamma
public Y Y Y Y
protected Y Y Y N
no modifier Y Y N N
private Y N N N
____________________ ____________________
Tips on Choosing an Access Level: If other programmers use your class, you want to ensure that errors from misuse cannot happen. Access levels can help you do this.
• Use the most restrictive access level that makes sense for a particular member. Use private unless you have a good reason not to.
• Avoid public fields except for constants. (Many of the examples in the tutorial use public fields. This may help to illustrate some points concisely, but is not recommended for production code.) Public fields tend to link you to a particular implementation and limit your flexibility in changing your code.


Understanding Instance and Class Members
In this section, we discuss the use of the static keyword to create fields and methods that belong to the class, rather than to an instance of the class.
Class Variables
When a number of objects are created from the same class blueprint, they each have their own distinct copies of instance variables. In the case of the Bicycle class, the instance variables are cadence, gear, and speed. Each Bicycle object has its own values for these variables, stored in different memory locations.
Sometimes, you want to have variables that are common to all objects. This is accomplished with the static modifier. Fields that have the static modifier in their declaration are called static fields or class variables. They are associated with the class, rather than with any object. Every instance of the class shares a class variable, which is in one fixed location in memory. Any object can change the value of a class variable, but class variables can also be manipulated without creating an instance of the class.
____________________ ____________________
Note: Public non-final statics are not recommended for applets because you can't make any assumptions on how the browser will implement the Java Virtual Machine (JVM). If your applet defines a public non-final static as a way of implementing a global variable, that variable may be clobbered when subsequent applets are launched. If your applet defines a public non-final static as a way of communicating between applets, you may find that the applets do not share the same JVM and are unable to communicate.
____________________ ____________________
For example, suppose you want to create a number of Bicycle objects and assign each a serial number, beginning with 1 for the first object. This ID number is unique to each object and is therefore an instance variable. At the same time, you need a field to keep track of how many Bicycle objects have been created so that you know what ID to assign to the next one. Such a field is not related to any individual object, but to the class as a whole. For this you need a class variable, numberOfBicycles, as follows:
public class Bicycle{

private int cadence;
private int gear;
private int speed;

// add an instance variable for the object ID
private int id;

// add a class variable for the number of Bicycle objects instantiated
private static int numberOfBicycles = 0;
......
}
Class variables are referenced by the class name itself, as in
Bicycle.numberOfBicy cles
This makes it clear that they are class variables.
____________________ ____________________
Note: You can also refer to static fields with an object reference like
myBike.numberOfBicyc les
but this is discouraged because it does not make it clear that they are class variables.
____________________ ____________________
You can use the Bicycle constructor to set the id instance variable and increment the numberOfBicycles class variable:
public class Bicycle{

private int cadence;
private int gear;
private int speed;
private int id;
private static int numberOfBicycles = 0;

public Bicycle(int startCadence, int startSpeed, int startGear){
gear = startGear;
cadence = startCadence;
speed = startSpeed;

// increment number of Bicycles and assign ID number
id = ++numberOfBicycles;
}

// new method to return the ID instance variable
public int getID() {
return id;
}
.....
}
Class Methods
The Java programming language supports static methods as well as static variables. Static methods, which have the static modifier in their declarations, should be invoked with the class name, without the need for creating an instance of the class, as in
ClassName.methodName (args)
____________________ ____________________
Note: You can also refer to static methods with an object reference like
instanceName.methodN ame(args)
but this is discouraged because it does not make it clear that they are class methods.
____________________ ____________________
A common use for static methods is to access static fields. For example, we could add a static method to the Bicycle class to access the numberOfBicycles static field:
public static int getNumberOfBicycles( ) {
return numberOfBicycles;
}
Not all combinations of instance and class variables and methods are allowed:
• Instance methods can access instance variables and instance methods directly.
• Instance methods can access class variables and class methods directly.
• Class methods can access class variables and class methods directly.
• Class methods cannot access instance variables or instance methods directly—they must use an object reference. Also, class methods cannot use the this keyword as there is no instance for this to refer to.
Constants
The static modifier, in combination with the final modifier, is also used to define constants. The final modifier indicates that the value of this field cannot change.
For example, the following variable declaration defines a constant named PI, whose value is an approximation of pi (the ratio of the circumference of a circle to its diameter):
static final double PI = 3.141592653589793;
Constants defined in this way cannot be reassigned, and it is a compile-time error if your program tries to do so. By convention, the names of constant values are spelled in uppercase letters. If the name is composed of more than one word, the words are separated by an underscore (_).
____________________ ____________________
Note: If a primitive type or a string is defined as a constant and the value is known at compile time, the compiler replaces the constant name everywhere in the code with its value. This is called a compile-time constant. If the value of the constant in the outside world changes (for example, if it is legislated that pi actually should be 3.975), you will need to recompile any classes that use this constant to get the current value.
____________________ ____________________
The Bicycle Class
After all the modifications made in this section, the Bicycle class is now:
public class Bicycle{

private int cadence;
private int gear;
private int speed;

private int id;

private static int numberOfBicycles = 0;


public Bicycle(int startCadence, int startSpeed, int startGear){
gear = startGear;
cadence = startCadence;
speed = startSpeed;

id = ++numberOfBicycles;
}

public int getID() {
return id;
}

public static int getNumberOfBicycles( ) {
return numberOfBicycles;
}

public int getCadence(){
return cadence;
}

public void setCadence(int newValue){
cadence = newValue;
}

public int getGear(){
return gear;
}

public void setGear(int newValue){
gear = newValue;
}

public int getSpeed(){
return speed;
}

public void applyBrake(int decrement){
speed -= decrement;
}

public void speedUp(int increment){
speed += increment;
}

Initializing Fields
As you have seen, you can often provide an initial value for a field in its declaration:
public class BedAndBreakfast {

public static int capacity = 10; //initialize to 10

private boolean full = false; //initialize to false
}
This works well when the initialization value is available and the initialization can be put on one line. However, this form of initialization has limitations because of its simplicity. If initialization requires some logic (for example, error handling or a for loop to fill a complex array), simple assignment is inadequate. Instance variables can be initialized in constructors, where error handling or other logic can be used. To provide the same capability for class variables, the Java programming language includes static initialization blocks.
____________________ ____________________
Note: It is not necessary to declare fields at the beginning of the class definition, although this is the most common practice. It is only necessary that they be declared and initialized before they are used.
____________________ ____________________
Static Initialization Blocks
A static initialization block is a normal block of code enclosed in braces, { }, and preceded by the static keyword. Here is an example:
static {

// whatever code is needed for initialization goes here
}
A class can have any number of static initialization blocks, and they can appear anywhere in the class body. The runtime system guarantees that static initialization blocks are called in the order that they appear in the source code.
There is an alternative to static blocks —you can write a private static method:
class Whatever {
public static varType myVar = initializeClassVaria ble();

private static varType initializeClassVaria ble() {

//initialization code goes here
}
}
The advantage of private static methods is that they can be reused later if you need to reinitialize the class variable.
Initializing Instance Members
Normally, you would put code to initialize an instance variable in a constructor. There are two alternatives to using a constructor to initialize instance variables: initializer blocks and final methods.
Initializer blocks for instance variables look just like static initializer blocks, but without the static keyword:
{

// whatever code is needed for initialization goes here
}
The Java compiler copies initializer blocks into every constructor. Therefore, this approach can be used to share a block of code between multiple constructors.
A final method cannot be overridden in a subclass. This is discussed in the lesson on interfaces and inheritance. Here is an example of using a final method for initializing an instance variable:
class Whatever {
private varType myVar = initializeInstanceVa riable();

protected final varType initializeInstanceVa riable() {

//initialization code goes here
}
}

This is especially useful if subclasses might want to reuse the initialization method. The method is final because calling non-final methods during instance initialization can cause problems. Joshua Bloch describes this in more detail in Effective Java.

Summary of Creating and Using Classes and Objects
A class declaration names the class and encloses the class body between braces. The class name can be preceded by modifiers. The class body contains fields, methods, and constructors for the class. A class uses fields to contain state information and uses methods to implement behavior. Constructors that initialize a new instance of a class use the name of the class and look like methods without a return type.
You control access to classes and members in the same way: by using an access modifier such as public in their declaration.
You specify a class variable or a class method by using the static keyword in the member's declaration. A member that is not declared as static is implicitly an instance member. Class variables are shared by all instances of a class and can be accessed through the class name as well as an instance reference. Instances of a class get their own copy of each instance variable, which must be accessed through an instance reference.
You create an object from a class by using the new operator and a constructor. The new operator returns a reference to the object that was created. You can assign the reference to a variable or use it directly.
Instance variables and methods that are accessible to code outside of the class that they are declared in can be referred to by using a qualified name. The qualified name of an instance variable looks like this:
objectReference.vari ableName
The qualified name of a method looks like this:
objectReference.meth odName(argumentList)

or

objectReference.meth odName()
The garbage collector automatically cleans up unused objects. An object is unused if the program holds no more references to it. You can explicitly drop a reference by setting the variable holding the reference to null.



Questions
1. Consider the following class:
public class IdentifyMyParts {
public static int x = 7;
public int y = 3;
}
a. What are the class variables?
b. What are the instance variables?
c. What is the output from the following code:
IdentifyMyParts a = new IdentifyMyParts();
IdentifyMyParts b = new IdentifyMyParts();
a.y = 5;
b.y = 6;
a.x = 1;
b.x = 2;
System.out.println("a.y = " + a.y);
System.out.println("b.y = " + b.y);
System.out.println("a.x = " + a.x);
System.out.println("b.x = " + b.x);
System.out.println("IdentifyMyParts.x = " + IdentifyMyParts.x);


Questions
1. What's wrong with the following program?
2. public class SomethingIsWrong {
3. public static void main(String[] args) {
4. Rectangle myRect;
5. myRect.width = 40;
6. myRect.height = 50;
7. System.out.println("myRect's area is " + myRect.area());
8. }
9. }


Nested Classes
The Java programming language allows you to define a class within another class. Such a class is called a nested class and is illustrated here:
class OuterClass {
...
class NestedClass {
...
}
}
____________________ ____________________
Terminology: Nested classes are divided into two categories: static and non-static. Nested classes that are declared static are simply called static nested classes. Non-static nested classes are called inner classes.
____________________ ____________________
class OuterClass {
...
static class StaticNestedClass {
...
}
class InnerClass {
...
}
}
A nested class is a member of its enclosing class. Non-static nested classes (inner classes) have access to other members of the enclosing class, even if they are declared private. Static nested classes do not have access to other members of the enclosing class. As a member of the OuterClass, a nested class can be declared private, public, protected, or package private. (Recall that outer classes can only be declared public or package private.)
Why Use Nested Classes?
There are several compelling reasons for using nested classes, among them:
• It is a way of logically grouping classes that are only used in one place.
• It increases encapsulation.
• Nested classes can lead to more readable and maintainable code.
Logical grouping of classes—If a class is useful to only one other class, then it is logical to embed it in that class and keep the two together. Nesting such "helper classes" makes their package more streamlined.
Increased encapsulation—Cons ider two top-level classes, A and B, where B needs access to members of A that would otherwise be declared private. By hiding class B within class A, A's members can be declared private and B can access them. In addition, B itself can be hidden from the outside world.
More readable, maintainable code—Nesting small classes within top-level classes places the code closer to where it is used.
Static Nested Classes
As with class methods and variables, a static nested class is associated with its outer class. And like static class methods, a static nested class cannot refer directly to instance variables or methods defined in its enclosing class — it can use them only through an object reference.
____________________ ____________________
Note: A static nested class interacts with the instance members of its outer class (and other classes) just like any other top-level class. In effect, a static nested class is behaviorally a top-level class that has been nested in another top-level class for packaging convenience.
____________________ ____________________
Static nested classes are accessed using the enclosing class name:
OuterClass.StaticNes tedClass
For example, to create an object for the static nested class, use this syntax:
OuterClass.StaticNes tedClass nestedObject = new OuterClass.StaticNes tedClass();
Inner Classes
As with instance methods and variables, an inner class is associated with an instance of its enclosing class and has direct access to that object's methods and fields. Also, because an inner class is associated with an instance, it cannot define any static members itself.
Objects that are instances of an inner class exist within an instance of the outer class. Consider the following classes:
class OuterClass {
...
class InnerClass {
...
}
}

An instance of InnerClass can exist only within an instance of OuterClass and has direct access to the methods and fields of its enclosing instance. The next figure illustrates this idea.

An InnerClass Exists Within an Instance of OuterClass
To instantiate an inner class, you must first instantiate the outer class. Then, create the inner object within the outer object with this syntax:
OuterClass.InnerClas s innerObject = outerObject.new InnerClass();
Additionally, there are two special kinds of inner classes: local classes and anonymous classes (also called anonymous inner classes). Both of these will be discussed briefly in the next section.


Inner Class Example
To see an inner class in use, let's first consider an array. In the following example, we will create an array, fill it with integer values and then output only values of even indices of the array in ascending order.
The DataStructure class below consists of:
• The DataStructure outer class, which includes methods to add an integer onto the array and print out values of even indices of the array.
• The InnerEvenIterator inner class, which is similar to a standard Java iterator. Iterators are used to step through a data structure and typically have methods to test for the last element, retrieve the current element, and move to the next element.
• A main method that instantiates a DataStructure object (ds) and uses it to fill the arrayOfInts array with integer values (0, 1, 2, 3, etc.), then calls a printEven method to print out values of even indices of arrayOfInts.

public class DataStructure {
//create an array
private final static int SIZE = 15;
private int[] arrayOfInts = new int[SIZE];

public DataStructure() {
//fill the array with ascending integer values
for (int i = 0; i < SIZE; i+ {
arrayOfInts[i] = i;
}
}

public void printEven() {
//print out values of even indices of the array
InnerEvenIterator iterator = this.new InnerEvenIterator();
while (iterator.hasNext()) {
System.out.println(i terator.getNext() + " ");
}
}

//inner class implements the Iterator pattern
private class InnerEvenIterator {
//start stepping through the array from the beginning
private int next = 0;

public boolean hasNext() {
//check if a current element is the last in the array
return (next <= SIZE - 1);
}

public int getNext() {
//record a value of an even index of the array
int retValue = arrayOfInts[next];
//get the next even element
next += 2;
return retValue;
}
}

public static void main(String s[]) {
//fill the array with integer values and print out only values of even indices
DataStructure ds = new DataStructure();
ds.printEven();
}
}
The output is:
0 2 4 6 8 10 12 14
Note that the InnerEvenIterator class refers directly to the arrayOfInts instance variable of the DataStructure object.
Inner classes can be used to implement helper classes like the one shown in the example above. If you plan on handling user-interface events, you will need to know how to use inner classes because the event-handling mechanism makes extensive use of them.
Local and Anonymous Inner Classes
There are two additional types of inner classes. You can declare an inner class within the body of a method. Such a class is known as a local inner class. You can also declare an inner class within the body of a method without naming it. These classes are known as anonymous inner classes. You will encounter such classes in advanced Java programming.
Modifiers
You can use the same modifiers for inner classes that you use for other members of the outer class. For example, you can use the access specifiers — private, public, and protected — to restrict access to inner classes, just as you do to other class members


Lesson: Interfaces and Inheritance
Interfaces
You saw an example of implementing an interface in the previous lesson. You can read more about interfaces here—what they are for, why you might want to write one, and how to write one.
Inheritance
This section describes the way in which you can derive one class from another. That is, how a subclass can inherit fields and methods from a superclass. You will learn that all classes are derived from the Object class, and how to modify the methods that a subclass inherits from superclasses. This section also covers interface-like abstract classes.


Interfaces
There are a number of situations in software engineering when it is important for disparate groups of programmers to agree to a "contract" that spells out how their software interacts. Each group should be able to write their code without any knowledge of how the other group's code is written. Generally speaking, interfaces are such contracts.
For example, imagine a futuristic society where computer-controlled robotic cars transport passengers through city streets without a human operator. Automobile manufacturers write software (Java, of course) that operates the automobile—stop, start, accelerate, turn left, and so forth. Another industrial group, electronic guidance instrument manufacturers, make computer systems that receive GPS (Global Positioning Satellite) position data and wireless transmission of traffic conditions and use that information to drive the car.
The auto manufacturers must publish an industry-standard interface that spells out in detail what methods can be invoked to make the car move (any car, from any manufacturer). The guidance manufacturers can then write software that invokes the methods described in the interface to command the car. Neither industrial group needs to know how the other group's software is implemented. In fact, each group considers its software highly proprietary and reserves the right to modify it at any time, as long as it continues to adhere to the published interface.
Interfaces in Java
In the Java programming language, an interface is a reference type, similar to a class, that can contain only constants, method signatures, and nested types. There are no method bodies. Interfaces cannot be instantiated—they can only be implemented by classes or extended by other interfaces. Extension is discussed later in this lesson.
Defining an interface is similar to creating a new class:
public interface OperateCar {

// constant declarations, if any

// method signatures
int turn(Direction direction, // An enum with values RIGHT, LEFT
double radius, double startSpeed, double endSpeed);
int changeLanes(Directio n direction, double startSpeed, double endSpeed);
int signalTurn(Direction direction, boolean signalOn);
int getRadarFront(double distanceToCar, double speedOfCar);
int getRadarRear(double distanceToCar, double speedOfCar);
......
// more method signatures
}
Note that the method signatures have no braces and are terminated with a semicolon.
To use an interface, you write a class that implements the interface. When an instantiable class implements an interface, it provides a method body for each of the methods declared in the interface. For example,
public class OperateBMW760i implements OperateCar {

// the OperateCar method signatures, with implementation --
// for example:
int signalTurn(Direction direction, boolean signalOn) {
//code to turn BMW's LEFT turn indicator lights on
//code to turn BMW's LEFT turn indicator lights off
//code to turn BMW's RIGHT turn indicator lights on
//code to turn BMW's RIGHT turn indicator lights off
}

// other members, as needed -- for example, helper classes
// not visible to clients of the interface

}
In the robotic car example above, it is the automobile manufacturers who will implement the interface. Chevrolet's implementation will be substantially different from that of Toyota, of course, but both manufacturers will adhere to the same interface. The guidance manufacturers, who are the clients of the interface, will build systems that use GPS data on a car's location, digital street maps, and traffic data to drive the car. In so doing, the guidance systems will invoke the interface methods: turn, change lanes, brake, accelerate, and so forth.
Interfaces as APIs
The robotic car example shows an interface being used as an industry standard Application Programming Interface (API). APIs are also common in commercial software products. Typically, a company sells a software package that contains complex methods that another company wants to use in its own software product. An example would be a package of digital image processing methods that are sold to companies making end-user graphics programs. The image processing company writes its classes to implement an interface, which it makes public to its customers. The graphics company then invokes the image processing methods using the signatures and return types defined in the interface. While the image processing company's API is made public (to its customers), its implementation of the API is kept as a closely guarded secret—in fact, it may revise the implementation at a later date as long as it continues to implement the original interface that its customers have relied on.
Interfaces and Multiple Inheritance
Interfaces have another very important role in the Java programming language. Interfaces are not part of the class hierarchy, although they work in combination with classes. The Java programming language does not permit multiple inheritance (inheritance is discussed later in this lesson), but interfaces provide an alternative.
In Java, a class can inherit from only one class but it can implement more than one interface. Therefore, objects can have multiple types: the type of their own class and the types of all the interfaces that they implement. This means that if a variable is declared to be the type of an interface, its value can reference any object that is instantiated from any class that implements the interface. This is discussed later in this lesson, in the section titled "Using an Interface as a Type."


Defining an Interface
An interface declaration consists of modifiers, the keyword interface, the interface name, a comma-separated list of parent interfaces (if any), and the interface body. For example:
public interface GroupedInterface extends Interface1,
Interface2, Interface3 {

// constant declarations
double E = 2.718282; // base of natural logarithms

// method signatures
void doSomething (int i, double x);
int doSomethingElse(Stri ng s);

}
The public access specifier indicates that the interface can be used by any class in any package. If you do not specify that the interface is public, your interface will be accessible only to classes defined in the same package as the interface.
An interface can extend other interfaces, just as a class can extend or subclass another class. However, whereas a class can extend only one other class, an interface can extend any number of interfaces. The interface declaration includes a comma-separated list of all the interfaces that it extends.
The Interface Body
The interface body contains method declarations for all the methods included in the interface. A method declaration within an interface is followed by a semicolon, but no braces, because an interface does not provide implementations for the methods declared within it. All methods declared in an interface are implicitly public, so the public modifier can be omitted.
An interface can contain constant declarations in addition to method declarations. All constant values defined in an interface are implicitly public, static, and final. Once again, these modifiers can be omitted.

Implementing an Interface
To declare a class that implements an interface, you include an implements clause in the class declaration. Your class can implement more than one interface, so the implements keyword is followed by a comma-separated list of the interfaces implemented by the class.
By convention, the implements clause follows the extends clause, if there is one.
A Sample Interface, Relatable
Consider an interface that defines how to compare the size of objects.

public interface Relatable {

// this (object calling isLargerThan) and
// other must be instances of the same class
// returns 1, 0, -1 if this is greater
// than, equal to, or less than other
public int isLargerThan(Relatab le other);

}
If you want to be able to compare the size of similar objects, no matter what they are, the class that instantiates them should implement Relatable.
Any class can implement Relatable if there is some way to compare the relative "size" of objects instantiated from the class. For strings, it could be number of characters; for books, it could be number of pages; for students, it could be weight; and so forth. For planar geometric objects, area would be a good choice (see the RectanglePlus class that follows), while volume would work for three-dimensional geometric objects. All such classes can implement the isLargerThan() method.
If you know that a class implements Relatable, then you know that you can compare the size of the objects instantiated from that class.
Implementing the Relatable Interface
Here is the Rectangle class that was presented in the Creating Objects section, rewritten to implement Relatable.
public class RectanglePlus implements Relatable {
public int width = 0;
public int height = 0;
public Point origin;

// four constructors
public RectanglePlus() {
origin = new Point(0, 0);
}
public RectanglePlus(Point p) {
origin = p;
}
public RectanglePlus(int w, int h) {
origin = new Point(0, 0);
width = w;
height = h;
}
public RectanglePlus(Point p, int w, int h) {
origin = p;
width = w;
height = h;
}

// a method for moving the rectangle
public void move(int x, int y) {
origin.x = x;
origin.y = y;
}

// a method for computing the area of the rectangle
public int getArea() {
return width * height;
}

// a method to implement Relatable
public int isLargerThan(Relatab le other) {
RectanglePlus otherRect = (RectanglePlus)other ;
if (this.getArea() < otherRect.getArea())
return -1;
else if (this.getArea() > otherRect.getArea())
return 1;
else
return 0;
}
}
Because RectanglePlus implements Relatable, the size of any two RectanglePlus objects can be compared.

Using an Interface as a Type
When you define a new interface, you are defining a new reference data type. You can use interface names anywhere you can use any other data type name. If you define a reference variable whose type is an interface, any object you assign to it must be an instance of a class that implements the interface.
As an example, here is a method for finding the largest object in a pair of objects, for any objects that are instantiated from a class that implements Relatable:
public Object findLargest(Object object1, Object object2) {
Relatable obj1 = (Relatable)object1;
Relatable obj2 = (Relatable)object2;
if ( (obj1).isLargerThan( obj2) > 0)
return object1;
else
return object2;
}
By casting object1 to a Relatable type, it can invoke the isLargerThan method.
If you make a point of implementing Relatable in a wide variety of classes, the objects instantiated from any of those classes can be compared with the findLargest() method—provided that both objects are of the same class. Similarly, they can all be compared with the following methods:
public Object findSmallest(Object object1, Object object2) {
Relatable obj1 = (Relatable)object1;
Relatable obj2 = (Relatable)object2;
if ( (obj1).isLargerThan( obj2) < 0)
return object1;
else
return object2;
}

public boolean isEqual(Object object1, Object object2) {
Relatable obj1 = (Relatable)object1;
Relatable obj2 = (Relatable)object2;
if ( (obj1).isLargerThan( obj2) == 0)
return true;
else
return false;
}
These methods work for any "relatable" objects, no matter what their class inheritance is. When they implement Relatable, they can be of both their own class (or superclass) type and a Relatable type. This gives them some of the advantages of multiple inheritance, where they can have behavior from both a superclass and an interface.

Summary of Interfaces
An interface defines a protocol of communication between two objects.
An interface declaration contains signatures, but no implementations, for a set of methods, and might also contain constant definitions.
A class that implements an interface must implement all the methods declared in the interface.
An interface name can be used anywhere a type can be used.


Inheritance
In the preceding lessons, you have seen inheritance mentioned several times. In the Java language, classes can be derived from other classes, thereby inheriting fields and methods from those classes.
____________________ ____________________
Definitions:
A class that is derived from another class is called a subclass (also a derived class, extended class, or child class). The class from which the subclass is derived is called a superclass (also a base class or a parent class).
Excepting Object, which has no superclass, every class has one and only one direct superclass (single inheritance). In the absence of any other explicit superclass, every class is implicitly a subclass of Object.
Classes can be derived from classes that are derived from classes that are derived from classes, and so on, and ultimately derived from the topmost class, Object. Such a class is said to be descended from all the classes in the inheritance chain stretching back to Object.
____________________ ____________________
The idea of inheritance is simple but powerful: When you want to create a new class and there is already a class that includes some of the code that you want, you can derive your new class from the existing class. In doing this, you can reuse the fields and methods of the existing class without having to write (and debug!) them yourself.
A subclass inherits all the members (fields, methods, and nested classes) from its superclass. Constructors are not members, so they are not inherited by subclasses, but the constructor of the superclass can be invoked from the subclass.
The Java Platform Class Hierarchy
The Object class, defined in the java.lang package, defines and implements behavior common to all classes—including the ones that you write. In the Java platform, many classes derive directly from Object, other classes derive from some of those classes, and so on, forming a hierarchy of classes.

All Classes in the Java Platform are Descendants of Object
At the top of the hierarchy, Object is the most general of all classes. Classes near the bottom of the hierarchy provide more specialized behavior.
An Example of Inheritance
Here is the sample code for a possible implementation of a Bicycle class that was presented in the Classes and Objects lesson:
public class Bicycle {

// the Bicycle class has three fields
public int cadence;
public int gear;
public int speed;

// the Bicycle class has one constructor
public Bicycle(int startCadence, int startSpeed, int startGear) {
gear = startGear;
cadence = startCadence;
speed = startSpeed;
}

// the Bicycle class has four methods
public void setCadence(int newValue) {
cadence = newValue;
}

public void setGear(int newValue) {
gear = newValue;
}

public void applyBrake(int decrement) {
speed -= decrement;
}

public void speedUp(int increment) {
speed += increment;
}

}
A class declaration for a MountainBike class that is a subclass of Bicycle might look like this:
public class MountainBike extends Bicycle {

// the MountainBike subclass adds one field
public int seatHeight;

// the MountainBike subclass has one constructor
public MountainBike(int startHeight, int startCadence, int startSpeed, int startGear) {
super(startCadence, startSpeed, startGear);
seatHeight = startHeight;
}

// the MountainBike subclass adds one method
public void setHeight(int newValue) {
seatHeight = newValue;
}

}
MountainBike inherits all the fields and methods of Bicycle and adds the field seatHeight and a method to set it. Except for the constructor, it is as if you had written a new MountainBike class entirely from scratch, with four fields and five methods. However, you didn't have to do all the work. This would be especially valuable if the methods in the Bicycle class were complex and had taken substantial time to debug.
What You Can Do in a Subclass
A subclass inherits all of the public and protected members of its parent, no matter what package the subclass is in. If the subclass is in the same package as its parent, it also inherits the package-private members of the parent. You can use the inherited members as is, replace them, hide them, or supplement them with new members:
• The inherited fields can be used directly, just like any other fields.
• You can declare a field in the subclass with the same name as the one in the superclass, thus hiding it (not recommended).
• You can declare new fields in the subclass that are not in the superclass.
• The inherited methods can be used directly as they are.
• You can write a new instance method in the subclass that has the same signature as the one in the superclass, thus overriding it.
• You can write a new static method in the subclass that has the same signature as the one in the superclass, thus hiding it.
• You can declare new methods in the subclass that are not in the superclass.
• You can write a subclass constructor that invokes the constructor of the superclass, either implicitly or by using the keyword super.
The following sections in this lesson will expand on these topics.
Private Members in a Superclass
A subclass does not inherit the private members of its parent class. However, if the superclass has public or protected methods for accessing its private fields, these can also be used by the subclass.
A nested class has access to all the private members of its enclosing class—both fields and methods. Therefore, a public or protected nested class inherited by a subclass has indirect access to all of the private members of the superclass.
Casting Objects
We have seen that an object is of the data type of the class from which it was instantiated. For example, if we write
public MountainBike myBike = new MountainBike();
then myBike is of type MountainBike.
MountainBike is descended from Bicycle and Object. Therefore, a MountainBike is a Bicycle and is also an Object, and it can be used wherever Bicycle or Object objects are called for.
The reverse is not necessarily true: a Bicycle may be a MountainBike, but it isn't necessarily. Similarly, an Object may be a Bicycle or a MountainBike, but it isn't necessarily.
Casting shows the use of an object of one type in place of another type, among the objects permitted by inheritance and implementations. For example, if we write
Object obj = new MountainBike();
then obj is both an Object and a Mountainbike (until such time as obj is assigned another object that is not a Mountainbike). This is called implicit casting.
If, on the other hand, we write
MountainBike myBike = obj;
we would get a compile-time error because obj is not known to the compiler to be a MountainBike. However, we can tell the compiler that we promise to assign a MountainBike to obj by explicit casting:
MountainBike myBike = (MountainBike)obj;
This cast inserts a runtime check that obj is assigned a MountainBike so that the compiler can safely assume that obj is a MountainBike. If obj is not a Mountainbike at runtime, an exception will be thrown.
____________________ ____________________
Note: You can make a logical test as to the type of a particular object using the instanceof operator. This can save you from a runtime error owing to an improper cast. For example:
if (obj instanceof MountainBike) {
MountainBike myBike = (MountainBike)obj;
}
Here the instanceof operator verifies that obj refers to a MountainBike so that we can make the cast with knowledge that there will be no runtime exception thrown

Overriding and Hiding Methods
Instance Methods
An instance method in a subclass with the same signature (name, plus the number and the type of its parameters) and return type as an instance method in the superclass overrides the superclass's method.
The ability of a subclass to override a method allows a class to inherit from a superclass whose behavior is "close enough" and then to modify behavior as needed. The overriding method has the same name, number and type of parameters, and return type as the method it overrides. An overriding method can also return a subtype of the type returned by the overridden method. This is called a covariant return type.
When overriding a method, you might want to use the @Override annotation that instructs the compiler that you intend to override a method in the superclass. If, for some reason, the compiler detects that the method does not exist in one of the superclasses, it will generate an error. For more information on @Override, see Annotations.
Class Methods
If a subclass defines a class method with the same signature as a class method in the superclass, the method in the subclass hides the one in the superclass.
The distinction between hiding and overriding has important implications. The version of the overridden method that gets invoked is the one in the subclass. The version of the hidden method that gets invoked depends on whether it is invoked from the superclass or the subclass. Let's look at an example that contains two classes. The first is Animal, which contains one instance method and one class method:
public class Animal {
public static void testClassMethod() {
System.out.println("The class method in Animal.");
}
public void testInstanceMethod() {
System.out.println("The instance method in Animal.");
}
}
The second class, a subclass of Animal, is called Cat:
public class Cat extends Animal {
public static void testClassMethod() {
System.out.println("The class method in Cat.");
}
public void testInstanceMethod() {
System.out.println("The instance method in Cat.");
}

public static void main(String[] args) {
Cat myCat = new Cat();
Animal myAnimal = myCat;
Animal.testClassMeth od();
myAnimal.testInstanc eMethod();
}
}
The Cat class overrides the instance method in Animal and hides the class method in Animal. The main method in this class creates an instance of Cat and calls testClassMethod() on the class and testInstanceMethod() on the instance.
The output from this program is as follows:
The class method in Animal.
The instance method in Cat.
As promised, the version of the hidden method that gets invoked is the one in the superclass, and the version of the overridden method that gets invoked is the one in the subclass.
Modifiers
The access specifier for an overriding method can allow more, but not less, access than the overridden method. For example, a protected instance method in the superclass can be made public, but not private, in the subclass.
You will get a compile-time error if you attempt to change an instance method in the superclass to a class method in the subclass, and vice versa.
Summary
The following table summarizes what happens when you define a method with the same signature as a method in a superclass.
Defining a Method with the Same Signature as a Superclass's Method
Superclass Instance Method Superclass Static Method
Subclass Instance Method Overrides Generates a compile-time error
Subclass Static Method Generates a compile-time error Hides
____________________ ____________________
Note: In a subclass, you can overload the methods inherited from the superclass. Such overloaded methods neither hide nor override the superclass methods—they are new methods, unique to the subclass.



Using the Keyword super
Accessing Superclass Members
If your method overrides one of its superclass's methods, you can invoke the overridden method through the use of the keyword super. You can also use super to refer to a hidden field (although hiding fields is discouraged). Consider this class, Superclass:
public class Superclass {

public void printMethod() {
System.out.println("Printed in Superclass.");
}
}
Here is a subclass, called Subclass, that overrides printMethod():
public class Subclass extends Superclass {

public void printMethod() { //overrides printMethod in Superclass
super.printMethod();
System.out.println("Printed in Subclass");
}
public static void main(String[] args) {

Subclass s = new Subclass();
s.printMethod();
}

}
Within Subclass, the simple name printMethod() refers to the one declared in Subclass, which overrides the one in Superclass. So, to refer to printMethod() inherited from Superclass, Subclass must use a qualified name, using super as shown. Compiling and executing Subclass prints the following:
Printed in Superclass.
Printed in Subclass
Subclass Constructors
The following example illustrates how to use the super keyword to invoke a superclass's constructor. Recall from the Bicycle example that MountainBike is a subclass of Bicycle. Here is the MountainBike (subclass) constructor that calls the superclass constructor and then adds initialization code of its own:
public MountainBike(int startHeight, int startCadence, int startSpeed, int startGear) {
super(startCadence, startSpeed, startGear);
seatHeight = startHeight;
}
Invocation of a superclass constructor must be the first line in the subclass constructor.
The syntax for calling a superclass constructor is
super();
--or--
super(parameter list);
With super(), the superclass no-argument constructor is called. With super(parameter list), the superclass constructor with a matching parameter list is called.
____________________ ____________________
Note: If a constructor does not explicitly invoke a superclass constructor, the Java compiler automatically inserts a call to the no-argument constructor of the superclass. If the super class does not have a no-argument constructor, you will get a compile-time error. Object does have such a constructor, so if Object is the only superclass, there is no problem.
____________________ ____________________
If a subclass constructor invokes a constructor of its superclass, either explicitly or implicitly, you might think that there will be a whole chain of constructors called, all the way back to the constructor of Object. In fact, this is the case. It is called constructor chaining, and you need to be aware of it when there is a long line of class descent.


Object as a Superclass
The Object class, in the java.lang package, sits at the top of the class hierarchy tree. Every class is a descendant, direct or indirect, of the Object class. Every class you use or write inherits the instance methods of Object. You need not use any of these methods, but, if you choose to do so, you may need to override them with code that is specific to your class. The methods inherited from Object that are discussed in this section are:
• protected Object clone() throws CloneNotSupportedExc eption
Creates and returns a copy of this object.
• public boolean equals(Object obj)
Indicates whether some other object is "equal to" this one.
• protected void finalize() throws Throwable
Called by the garbage collector on an object when garbage
collection determines that there are no more references to the object
• public final Class getClass()
Returns the runtime class of an object.
• public int hashCode()
Returns a hash code value for the object.
• public String toString()
Returns a string representation of the object.
The notify, notifyAll, and wait methods of Object all play a part in synchronizing the activities of independently running threads in a program, which is discussed in a later lesson and won't be covered here. There are five of these methods:
• public final void notify()
• public final void notifyAll()
• public final void wait()
• public final void wait(long timeout)
• public final void wait(long timeout, int nanos)
____________________ ____________________
Note: There are some subtle aspects to a number of these methods, especially the clone method. You can get information on the correct usage of these methods in the book Effective Java by Josh Bloch.
____________________ ____________________
The clone() Method
If a class, or one of its superclasses, implements the Cloneable interface, you can use the clone() method to create a copy from an existing object. To create a clone, you write:
aCloneableObject.clo ne();
Object's implementation of this method checks to see whether the object on which clone() was invoked implements the Cloneable interface. If the object does not, the method throws a CloneNotSupportedExc eption exception. Exception handling will be covered in a later lesson. For the moment, you need to know that clone() must be declared as
protected Object clone() throws CloneNotSupportedExc eption
-- or --
public Object clone() throws CloneNotSupportedExc eption
if you are going to write a clone() method to override the one in Object.
If the object on which clone() was invoked does implement the Cloneable interface, Object's implementation of the clone() method creates an object of the same class as the original object and initializes the new object's member variables to have the same values as the original object's corresponding member variables.
The simplest way to make your class cloneable is to add implements Cloneable to your class's declaration. then your objects can invoke the clone() method.
For some classes, the default behavior of Object's clone() method works just fine. If, however, an object contains a reference to an external object, say ObjExternal, you may need to override clone() to get correct behavior. Otherwise, a change in ObjExternal made by one object will be visible in its clone also. This means that the original object and its clone are not independent—to decouple them, you must override clone() so that it clones the object and ObjExternal. Then the original object references ObjExternal and the clone references a clone of ObjExternal, so that the object and its clone are truly independent.
The equals() Method
The equals() method compares two objects for equality and returns true if they are equal. The equals() method provided in the Object class uses the identity operator (==) to determine whether two objects are equal. For primitive data types, this gives the correct result. For objects, however, it does not. The equals() method provided by Object tests whether the object references are equal—that is, if the objects compared are the exact same object.
To test whether two objects are equal in the sense of equivalency (containing the same information), you must override the equals() method. Here is an example of a Book class that overrides equals():
public class Book {
...
public boolean equals(Object obj) {
if (obj instanceof Book)
return ISBN.equals((Book)ob j.getISBN());
else
return false;
}
}
Consider this code that tests two instances of the Book class for equality:
Book firstBook = new Book("0201914670"); //Swing Tutorial, 2nd edition
Book secondBook = new Book("0201914670");
if (firstBook.equals(se condBook)) {
System.out.println("objects are equal");
} else {
System.out.println("objects are not equal");
}
This program displays objects are equal even though firstBook and secondBook reference two distinct objects. They are considered equal because the objects compared contain the same ISBN number.
You should always override the equals() method if the identity operator is not appropriate for your class.
____________________ ____________________
Note: If you override equals(), you must override hashCode() as well.
____________________ ____________________
The finalize() Method
The Object class provides a callback method, finalize(), that may be invoked on an object when it becomes garbage. Object's implementation of finalize() does nothing—you can override finalize() to do cleanup, such as freeing resources.
The finalize() method may be called automatically by the system, but when it is called, or even if it is called, is uncertain. Therefore, you should not rely on this method to do your cleanup for you. For example, if you don't close file descriptors in your code after performing I/O and you expect finalize() to close them for you, you may run out of file descriptors.
The getClass() Method
You cannot override getClass.
The getClass() method returns a Class object, which has methods you can use to get information about the class, such as its name (getSimpleName()), its superclass (getSuperclass()), and the interfaces it implements (getInterfaces()). For example, the following method gets and displays the class name of an object:
void printClassName(Objec t obj) {
System.out.println("The object's class is "
obj.getClass().getSi mpleName());
}
The Class class, in the java.lang package, has a large number of methods (more than 50). For example, you can test to see if the class is an annotation (isAnnotation()), an interface (isInterface()), or an enumeration (isEnum()). You can see what the object's fields are (getFields()) or what its methods are (getMethods()), and so on.
The hashCode() Method
The value returned by hashCode() is the object's hash code, which is the object's memory address in hexadecimal.
By definition, if two objects are equal, their hash code must also be equal. If you override the equals() method, you change the way two objects are equated and Object's implementation of hashCode() is no longer valid. Therefore, if you override the equals() method, you must also override the hashCode() method as well.
The toString() Method
You should always consider overriding the toString() method in your classes.
The Object's toString() method returns a String representation of the object, which is very useful for debugging. The String representation for an object depends entirely on the object, which is why you need to override toString() in your classes.
You can use toString() along with System.out.println() to display a text representation of an object, such as an instance of Book:
System.out.println(f irstBook.toString()) ;
which would, for a properly overridden toString() method, print something useful, like this:


Abstract Methods and Classes
An abstract class is a class that is declared abstract—it may or may not include abstract methods. Abstract classes cannot be instantiated, but they can be subclassed.
An abstract method is a method that is declared without an implementation (without braces, and followed by a semicolon), like this:
abstract void moveTo(double deltaX, double deltaY);
If a class includes abstract methods, the class itself must be declared abstract, as in:
public abstract class GraphicObject {
// declare fields
// declare non-abstract methods
abstract void draw();
}
When an abstract class is subclassed, the subclass usually provides implementations for all of the abstract methods in its parent class. However, if it does not, the subclass must also be declared abstract.
____________________ ____________________
Note: All of the methods in an interface (see the Interfaces section) are implicitly abstract, so the abstract modifier is not used with interface methods (it could be—it's just not necessary).
____________________ ____________________
Abstract Classes versus Interfaces
Unlike interfaces, abstract classes can contain fields that are not static and final, and they can contain implemented methods. Such abstract classes are similar to interfaces, except that they provide a partial implementation, leaving it to subclasses to complete the implementation. If an abstract class contains only abstract method declarations, it should be declared as an interface instead.
Multiple interfaces can be implemented by classes anywhere in the class hierarchy, whether or not they are related to one another in any way. Think of Comparable or Cloneable, for example.
By comparison, abstract classes are most commonly subclassed to share pieces of implementation. A single abstract class is subclassed by similar classes that have a lot in common (the implemented parts of the abstract class), but also have some differences (the abstract methods).
An Abstract Class Example
In an object-oriented drawing application, you can draw circles, rectangles, lines, Bezier curves, and many other graphic objects. These objects all have certain states (for example: position, orientation, line color, fill color) and behaviors (for example: moveTo, rotate, resize, draw) in common. Some of these states and behaviors are the same for all graphic objects—for example: position, fill color, and moveTo. Others require different implementations—fo r example, resize or draw. All GraphicObjects must know how to draw or resize themselves; they just differ in how they do it. This is a perfect situation for an abstract superclass. You can take advantage of the similarities and declare all the graphic objects to inherit from the same abstract parent object—for example, GraphicObject, as shown in the following figure.

Classes Rectangle, Line, Bezier, and Circle inherit from GraphicObject
First, you declare an abstract class, GraphicObject, to provide member variables and methods that are wholly shared by all subclasses, such as the current position and the moveTo method. GraphicObject also declares abstract methods for methods, such as draw or resize, that need to be implemented by all subclasses but must be implemented in different ways. The GraphicObject class can look something like this:
abstract class GraphicObject {
int x, y;
...
void moveTo(int newX, int newY) {
...
}
abstract void draw();
abstract void resize();
}
Each non-abstract subclass of GraphicObject, such as Circle and Rectangle, must provide implementations for the draw and resize methods:
class Circle extends GraphicObject {
void draw() {
...
}
void resize() {
...
}
}
class Rectangle extends GraphicObject {
void draw() {
...
}
void resize() {
...
}
}
When an Abstract Class Implements an Interface
In the section on Interfaces , it was noted that a class that implements an interface must implement all of the interface's methods. It is possible, however, to define a class that does not implement all of the interface methods, provided that the class is declared to be abstract. For example,
abstract class X implements Y {
// implements all but one method of Y
}

class XX extends X {
// implements the remaining method in Y
}
In this case, class X must be abstract because it does not fully implement Y, but class XX does, in fact, implement Y.
Class Members
An abstract class may have static fields and static methods. You can use these static members with a class reference—for example, AbstractClass.static Method()—as you would with any other class.


Summary of Inheritance
Except for the Object class, a class has exactly one direct superclass. A class inherits fields and methods from all its superclasses, whether direct or indirect. A subclass can override methods that it inherits, or it can hide fields or methods that it inherits. (Note that hiding fields is generally bad programming practice.)
The table in Overriding and Hiding Methods section shows the effect of declaring a method with the same signature as a method in the superclass.
The Object class is the top of the class hierarchy. All classes are descendants from this class and inherit methods from it. Useful methods inherited from Object include toString(), equals(), clone(), and getClass().
You can prevent a class from being subclassed by using the final keyword in the class's declaration. Similarly, you can prevent a method from being overridden by subclasses by declaring it as a final method.
An abstract class can only be subclassed; it cannot be instantiated. An abstract class can contain abstract methods—methods that are declared but not implemented. Subclasses then provide the implementations for the abstract methods.
Questions
1. Consider the following two classes:
public class ClassA {
public void methodOne(int i) {
}
public void methodTwo(int i) {
}
public static void methodThree(int i) {
}
public static void methodFour(int i) {
}
}

public class ClassB extends ClassA {
public static void methodOne(int i) {
}
public void methodTwo(int i) {
}
public void methodThree(int i) {
}
public static void methodFour(int i) {
}
}
a. Which method overrides a method in the superclass?
b. Which method hides a method in the superclass?
c. What do the other methods do?

Lesson: Numbers and Strings
Numbers
This section begins with a discussion of the Number class (in the java.lang package) and its subclasses. In particular, this section talks about the situations where you would use instantiations of these classes rather than the primitive data types. Additionally, this section talks about other classes you might need to work with numbers, such as formatting or using mathematical functions to complement the operators built into the language.
Strings
Strings, which are widely used in Java programming, are a sequence of characters. In the Java programming language, strings are objects. This section describes using the String class to create and manipulate strings. It also compares the String and StringBuilder classes.

Numbers
This section begins with a discussion of the Number class in the java.lang package, its subclasses, and the situations where you would use instantiations of these classes rather than the primitive number types.
This section also presents the PrintStream and DecimalFormat classes, which provide methods for writing formatted numerical output.
Finally, the Math class in java.lang is discussed. It contains mathematical functions to complement the operators built into the language. This class has methods for the trigonometric functions, exponential functions, and so forth.

The Numbers Classes
When working with numbers, most of the time you use the primitive types in your code. For example:
int i = 500;
float gpa = 3.65f;
byte mask = 0xff;
There are, however, reasons to use objects in place of primitives, and the Java platform provides wrapper classes for each of the primitive data types. These classes "wrap" the primitive in an object. Often, the wrapping is done by the compiler—if you use a primitive where an object is expected, the compiler boxes the primitive in its wrapper class for you. Similarly, if you use a number object when a primitive is expected, the compiler unboxes the object for you.
Here is an example of boxing and unboxing:
Integer x, y;
x = 12;
y = 15;
System.out.println(x +y);
When x and y are assigned integer values, the compiler boxes the integers because x and y are integer objects. In the println() statement, x and y are unboxed so that they can be added as integers.
All of the numeric wrapper classes are subclasses of the abstract class Number:

____________________ ____________________
Note: There are four other subclasses of Number that are not discussed here. BigDecimal and BigInteger are used for high-precision calculations. AtomicInteger and AtomicLong are used for multi-threaded applications.
____________________ ____________________
There are three reasons that you might use a Number object rather than a primitive:
1. As an argument of a method that expects an object (often used when manipulating collections of numbers).
2. To use constants defined by the class, such as MIN_VALUE and MAX_VALUE, that provide the upper and lower bounds of the data type.
3. To use class methods for converting values to and from other primitive types, for converting to and from strings, and for converting between number systems (decimal, octal, hexadecimal, binary).
The following table lists the instance methods that all the subclasses of the Number class implement.
Methods Implemented by all Subclasses of Number
Method Description
byte byteValue()
short shortValue()
int intValue()
long longValue()
float floatValue()
double doubleValue() Converts the value of this Number object to the primitive data type returned.
int compareTo(Byte anotherByte)
int compareTo(Double anotherDouble)
int compareTo(Float anotherFloat)
int compareTo(Integer anotherInteger)
int compareTo(Long anotherLong)
int compareTo(Short anotherShort) Compares this Number object to the argument.
boolean equals(Object obj) Determines whether this number object is equal to the argument.
The methods return true if the argument is not null and is an object of the same type and with the same numeric value.
There are some extra requirements for Double and Float objects that are described in the Java API documentation.
Each Number class contains other methods that are useful for converting numbers to and from strings and for converting between number systems. The following table lists these methods in the Integer class. Methods for the other Number subclasses are similar:
Conversion Methods, Integer Class
Method Description
static Integer decode(String s) Decodes a string into an integer. Can accept string representations of decimal, octal, or hexadecimal numbers as input.
static int parseInt(String s) Returns an integer (decimal only).
static int parseInt(String s, int radix) Returns an integer, given a string representation of decimal, binary, octal, or hexadecimal (radix equals 10, 2, 8, or 16 respectively) numbers as input.
String toString() Returns a String object representing the value of this Integer.
static String toString(int i) Returns a String object representing the specified integer.
static Integer valueOf(int i) Returns an Integer object holding the value of the specified primitive.
static Integer valueOf(String s) Returns an Integer object holding the value of the specified string representation.
static Integer valueOf(String s, int radix) Returns an Integer object holding the integer value of the specified string representation, parsed with the value of radix. For example, if s = "333" and radix = 8, the method returns the base-ten integer equivalent of the octal number 333.


Beyond Basic Arithmetic
The Java programming language supports basic arithmetic with its arithmetic operators: +, -, *, /, and %. The Math class in the java.lang package provides methods and constants for doing more advanced mathematical computation.
The methods in the Math class are all static, so you call them directly from the class, like this:
Math.cos(angle);
____________________ ____________________
Note : Using the static import language feature, you don't have to write Math in front of every math function:
import static java.lang.Math.*;
This allows you to invoke the Math class methods by their simple names. For example:
cos(angle);
____________________ ____________________
Constants and Basic Methods
The Math class includes two constants:
• Math.E, which is the base of natural logarithms, and
• Math.PI, which is the ratio of the circumference of a circle to its diameter.
The Math class also includes more than 40 static methods. The following table lists a number of the basic methods.
Basic Math Methods
Method Description
double abs(double d)
float abs(float f)
int abs(int i)
long abs(long lng) Returns the absolute value of the argument.
double ceil(double d) Returns the smallest integer that is greater than or equal to the argument. Returned as a double.
double floor(double d) Returns the largest integer that is less than or equal to the argument. Returned as a double.
double rint(double d) Returns the integer that is closest in value to the argument. Returned as a double.
long round(double d)
int round(float f) Returns the closest long or int, as indicated by the method's return type, to the argument.
double min(double arg1, double arg2)
float min(float arg1, float arg2)
int min(int arg1, int arg2)
long min(long arg1, long arg2) Returns the smaller of the two arguments.
double madouble arg1, double arg2)
float mafloat arg1, float arg2)
int maint arg1, int arg2)
long malong arg1, long arg2) Returns the larger of the two arguments.
The following program, BasicMathDemo , illustrates how to use some of these methods:

public class BasicMathDemo {
public static void main(String[] args) {
double a = -191.635;
double b = 43.74;
int c = 16, d = 45;

System.out.printf("The absolute value of %.3f is %.3f%n", a, Math.abs(a));
System.out.printf("The ceiling of %.2f is %.0f%n", b, Math.ceil(b));
System.out.printf("The floor of %.2f is %.0f%n", b, Math.floor(b));
System.out.printf("The rint of %.2f is %.0f%n", b, Math.rint(b));
System.out.printf("The max of %d and %d is %d%n",c, d, Math.mac, d));
System.out.printf("The min of of %d and %d is %d%n",c, d, Math.min(c, d));


}
}
Here's the output from this program:
The absolute value of -191.635 is 191.635
The ceiling of 43.74 is 44
The floor of 43.74 is 43
The rint of 43.74 is 44
The max of 16 and 45 is 45
The min of 16 and 45 is 16
Exponential and Logarithmic Methods
The next table lists exponential and logarithmic methods of the Math class.
Exponential and Logarithmic Methods
Method Description
double exp(double d) Returns the base of the natural logarithms, e, to the power of the argument.
double log(double d) Returns the natural logarithm of the argument.
double pow(double base, double exponent) Returns the value of the first argument raised to the power of the second argument.
double sqrt(double d) Returns the square root of the argument.
The following program, ExponentialDemo , displays the value of e, then calls each of the methods listed in the previous table on arbitrarily chosen numbers:

public class ExponentialDemo {
public static void main(String[] args) {
double x = 11.635;
double y = 2.76;

System.out.printf("The value of e is %.4f%n", Math.E);
System.out.printf("exp(%.3f) is %.3f%n", x, Math.exp(x));
System.out.printf("log(%.3f) is %.3f%n", x, Math.log(x));
System.out.printf("pow(%.3f, %.3f) is %.3f%n", x, y, Math.pow(x, y));
System.out.printf("sqrt(%.3f) is %.3f%n", x, Math.sqrt(x));
}
}
Here's the output you'll see when you run ExponentialDemo:
The value of e is 2.7183
exp(11.635) is 112983.831
log(11.635) is 2.454
pow(11.635, 2.760) is 874.008
sqrt(11.635) is 3.411
Trigonometric Methods
The Math class also provides a collection of trigonometric functions, which are summarized in the following table. The value passed into each of these methods is an angle expressed in radians. You can use the toRadians method to convert from degrees to radians.
Trigonometric Methods
Method Description
double sin(double d) Returns the sine of the specified double value.
double cos(double d) Returns the cosine of the specified double value.
double tan(double d) Returns the tangent of the specified double value.
double asin(double d) Returns the arcsine of the specified double value.
double acos(double d) Returns the arccosine of the specified double value.
double atan(double d) Returns the arctangent of the specified double value.
double atan2(double y, double x) Converts rectangular coordinates (x, y) to polar coordinate (r, theta) and returns theta.
double toDegrees(double d)
double toRadians(double d) Converts the argument to degrees or radians.
Here's a program, TrigonometricDemo , that uses each of these methods to compute various trigonometric values for a 45-degree angle:

public class TrigonometricDemo {
public static void main(String[] args) {
double degrees = 45.0;
double radians = Math.toRadians(degre es);

System.out.format("The value of pi is %.4f%n", Math.PI);
System.out.format("The sine of %.1f degrees is %.4f%n", degrees,
Math.sin(radians));
System.out.format("The cosine of %.1f degrees is %.4f%n", degrees,
Math.cos(radians));
System.out.format("The tangent of %.1f degrees is %.4f%n", degrees,
Math.tan(radians));
System.out.format("The arcsine of %.4f is %.4f degrees %n",
Math.sin(radians),
Math.toDegrees(Math. asin(Math.sin(radian s))));
System.out.format("The arccosine of %.4f is %.4f degrees %n",
Math.cos(radians),
Math.toDegrees(Math. acos(Math.cos(radian s))));
System.out.format("The arctangent of %.4f is %.4f degrees %n",
Math.tan(radians),
Math.toDegrees(Math. atan(Math.tan(radian s))));

}
}


The output of this program is as follows:
The value of pi is 3.1416
The sine of 45.0 degrees is 0.7071
The cosine of 45.0 degrees is 0.7071
The tangent of 45.0 degrees is 1.0000
The arcsine of 0.7071 is 45.0000 degrees
The arccosine of 0.7071 is 45.0000 degrees
The arctangent of 1.0000 is 45.0000 degrees
Random Numbers
The random() method returns a pseudo-randomly selected number between 0.0 and 1.0. The range includes 0.0 but not 1.0. In other words: 0.0 <= Math.random() < 1.0. To get a number in a different range, you can perform arithmetic on the value returned by the random method. For example, to generate an integer between 0 and 9, you would write:
int number = (int)(Math.random() * 10);
By multiplying the value by 10, the range of possible values becomes 0.0 <= number < 10.0.
Using Math.random works well when you need to generate a single random number. If you need to generate a series of random numbers, you should create an instance of java.util.Random and invoke methods on that object to generate numbers.


Characters
Most of the time, if you are using a single character value, you will use the primitive char type. For example:
char ch = 'a';
char uniChar = '\u039A'; // Unicode for uppercase Greek omega character
char[] charArray ={ 'a', 'b', 'c', 'd', 'e' }; // an array of chars
There are times, however, when you need to use a char as an object—for example, as a method argument where an object is expected. The Java programming language provides a wrapper class that "wraps" the char in a Character object for this purpose. An object of type Character contains a single field, whose type is char. This Character class also offers a number of useful class (i.e., static) methods for manipulating characters.
You can create a Character object with the Character constructor:
Character ch = new Character('a');
The Java compiler will also create a Character object for you under some circumstances. For example, if you pass a primitive char into a method that expects an object, the compiler automatically converts the char to a Character for you. This feature is called autoboxing—or unboxing, if the conversion goes the other way.
Here is an example of boxing,
Character ch = 'a'; // the primitive char 'a' is boxed into the Character object ch
and here is an example of both boxing and unboxing,
Character test(Character c) {...} // method parameter and return type = Character object

char c = test('x'); // primitive 'x' is boxed for method test, return is unboxed to char 'c'
____________________ ____________________
Note: The Character class is immutable, so that once it is created, a Character object cannot be changed.
____________________ ____________________
The following table lists some of the most useful methods in the Character class, but is not exhaustive. For a complete listing of all methods in this class (there are more than 50), refer to the java.lang.Character API specification.
Useful Methods in the Character Class
Method Description
boolean isLetter(char ch)
boolean isDigit(char ch) Determines whether the specified char value is a letter or a digit, respectively.
boolean isWhitespace(char ch) Determines whether the specified char value is white space.
boolean isUpperCase(char ch)
boolean isLowerCase(char ch) Determines whether the specified char value is uppercase or lowercase, respectively.
char toUpperCase(char ch)
char toLowerCase(char ch) Returns the uppercase or lowercase form of the specified char value.
toString(char ch) Returns a String object representing the specified character value—that is, a one-character string.
Escape Sequences
A character preceded by a backslash (\) is an escape sequence and has special meaning to the compiler. The newline character (\n) has been used frequently in this tutorial in System.out.println() statements to advance to the next line after the string is printed. The following table shows the Java escape sequences:
Escape Sequences
Escape Sequence Description
\t Insert a tab in the text at this point.
\b Insert a backspace in the text at this point.
\n Insert a newline in the text at this point.
\r Insert a carriage return in the text at this point.
\f Insert a formfeed in the text at this point.
\' Insert a single quote character in the text at this point.
\" Insert a double quote character in the text at this point.
\\ Insert a backslash character in the text at this point.
When an escape sequence is encountered in a print statement, the compiler interprets it accordingly. For example, if you want to put quotes within quotes you must use the escape sequence, \", on the interior quotes. To print the sentence
She said "Hello!" to me.
you would write
System.out.println("She said \"Hello!\" to me.");

Strings
Strings, which are widely used in Java programming, are a sequence of characters. In the Java programming language, strings are objects.
The Java platform provides the String class to create and manipulate strings.
Creating Strings
The most direct way to create a string is to write:
String greeting = "Hello world!";
In this case, "Hello world!" is a string literal—a series of characters in your code that is enclosed in double quotes. Whenever it encounters a string literal in your code, the compiler creates a String object with its value—in this case, Hello world!.
As with any other object, you can create String objects by using the new keyword and a constructor. The String class has 11 constructors that allow you to provide the initial value of the string using different sources, such as an array of characters:
char[] helloArray = { 'h', 'e', 'l', 'l', 'o', '.'};
String helloString = new String(helloArray);
System.out.println(h elloString);
The last line of this code snippet displays hello.
____________________ ____________________
Note: The String class is immutable, so that once it is created a String object cannot be changed. The String class has a number of methods, some of which will be discussed below, that appear to modify strings. Since strings are immutable, what these methods really do is create and return a new string that contains the result of the operation.
____________________ ____________________
String Length
Methods used to obtain information about an object are known as accessor methods. One accessor method that you can use with strings is the length() method, which returns the number of characters contained in the string object. After the following two lines of code have been executed, len equals 17:
String palindrome = "Dot saw I was Tod";
int len = palindrome.length();
A palindrome is a word or sentence that is symmetric—it is spelled the same forward and backward, ignoring case and punctuation. Here is a short and inefficient program to reverse a palindrome string. It invokes the String method charAt(i), which returns the ith character in the string, counting from 0.

public class StringDemo {
public static void main(String[] args) {
String palindrome = "Dot saw I was Tod";
int len = palindrome.length();
char[] tempCharArray = new char[len];
char[] charArray = new char[len];

// put original string in an array of chars
for (int i = 0; i < len; i+ {
tempCharArray[i] = palindrome.charAt(i) ;
}

// reverse array of chars
for (int j = 0; j < len; j+ {
charArray[j] = tempCharArray[len - 1 - j];
}

String reversePalindrome = new String(charArray);
System.out.println(r eversePalindrome);
}
}
Running the program produces this output:
doT saw I was toD
To accomplish the string reversal, the program had to convert the string to an array of characters (first for loop), reverse the array into a second array (second for loop), and then convert back to a string. The String class includes a method, getChars(), to convert a string, or a portion of a string, into an array of characters so we could replace the first for loop in the program above with
palindrome.getChars( 0, len, tempCharArray, 0);
Concatenating Strings
The String class includes a method for concatenating two strings:
string1.concat(strin g2);
This returns a new string that is string1 with string2 added to it at the end.
You can also use the concat() method with string literals, as in:
"My name is ".concat("Rumplestiltskin");
Strings are more commonly concatenated with the + operator, as in
"Hello," + " world" + "!"
which results in
"Hello, world!"
The + operator is widely used in print statements. For example:
String string1 = "saw I was ";
System.out.println("Dot " + string1 + "Tod");
which prints
Dot saw I was Tod
Such a concatenation can be a mixture of any objects. For each object that is not a String, its toString() method is called to convert it to a String.
____________________ ____________________
Note: The Java programming language does not permit literal strings to span lines in source files, so you must use the + concatenation operator at the end of each line in a multi-line string. For example,
String quote = "Now is the time for all good " +
"men to come to the aid of their country.";
Breaking strings between lines using the + concatenation operator is, once again, very common in print statements.
____________________ ____________________
Creating Format Strings
You have seen the use of the printf() and format() methods to print output with formatted numbers. The String class has an equivalent class method, format(), that returns a String object rather than a PrintStream object.
Using String's static format() method allows you to create a formatted string that you can reuse, as opposed to a one-time print statement. For example, instead of
System.out.printf("The value of the float variable is %f, while the value of the " +
"integer variable is %d, and the string is %s", floatVar, intVar, stringVar);
you can write
String fs;
fs = String.format("The value of the float variable is %f, while the value of the " +
"integer variable is %d, and the string is %s", floatVar, intVar, stringVar);
System.out.println(f s);



Converting Between Numbers and Strings
Converting Strings to Numbers
Frequently, a program ends up with numeric data in a string object—a value entered by the user, for example.
The Number subclasses that wrap primitive numeric types ( Byte, Integer, Double, Float, Long, and Short) each provide a class method named valueOf that converts a string to an object of that type. Here is an example, ValueOfDemo , that gets two strings from the command line, converts them to numbers, and performs arithmetic operations on the values:

public class ValueOfDemo {
public static void main(String[] args) {

//this program requires two arguments on the command line
if (args.length == 2) {
//convert strings to numbers
float a = (Float.valueOf(args[0]) ).floatValue();
float b = (Float.valueOf(args[1]) ).floatValue();

//do some arithmetic
System.out.println("a + b = " + (a + b) );
System.out.println("a - b = " + (a - b) );
System.out.println("a * b = " + (a * b) );
System.out.println("a / b = " + (a / b) );
System.out.println("a % b = " + (a % b) );
} else {
System.out.println("This program requires two command-line arguments.");
}
}
}
The following is the output from the program when you use 4.5 and 87.2 for the command-line arguments:
a + b = 91.7
a - b = -82.7
a * b = 392.4
a / b = 0.0516055
a % b = 4.5
____________________ ____________________
Note: Each of the Number subclasses that wrap primitive numeric types also provides a parseXXXX() method (for example, parseFloat()) that can be used to convert strings to primitive numbers. Since a primitive type is returned instead of an object, the parseFloat() method is more direct than the valueOf() method. For example, in the ValueOfDemo program, we could use:
float a = Float.parseFloat(arg s[0]);
float b = Float.parseFloat(arg s[1]);
____________________ ____________________
Converting Numbers to Strings
Sometimes you need to convert a number to a string because you need to operate on the value in its string form. There are several easy ways to convert a number to a string:
int i;
String s1 = "" + i; //Concatenate "i" with an empty string;
//conversion is handled for you.
or
String s2 = String.valueOf(i); //The valueOf class method.
Each of the Number subclasses includes a class method, toString(), that will convert its primitive type to a string. For example:
int i;
double d;
String s3 = Integer.toString(i);
String s4 = Double.toString(d);
The ToStringDemo example uses the toString method to convert a number to a string. The program then uses some string methods to compute the number of digits before and after the decimal point:

public class ToStringDemo {

public static void main(String[] args) {
double d = 858.48;
String s = Double.toString(d);

int dot = s.indexOf('.');

System.out.println(d ot + " digits before decimal point.");
System.out.println( (s.length() - dot - 1) +
" digits after decimal point.");
}
}
The output of this program is:
3 digits before decimal point.
2 digits after decimal point.


Manipulating Characters in a String
The String class has a number of methods for examining the contents of strings, finding characters or substrings within a string, changing case, and other tasks.
Getting Characters and Substrings by Index
You can get the character at a particular index within a string by invoking the charAt() accessor method. The index of the first character is 0, while the index of the last character is length()-1. For example, the following code gets the character at index 9 in a string:
String anotherPalindrome = "Niagara. O roar again!";
char aChar = anotherPalindrome.ch arAt(9);
Indices begin at 0, so the character at index 9 is 'O', as illustrated in the following figure:

If you want to get more than one consecutive character from a string, you can use the substring method. The substring method has two versions, as shown in the following table:
The substring Methods in the String Class
Method Description
String substring(int beginIndex, int endIndex) Returns a new string that is a substring of this string. The first integer argument specifies the index of the first character. The second integer argument is the index of the last character + 1.
String substring(int beginIndex) Returns a new string that is a substring of this string. The integer argument specifies the index of the first character. Here, the returned substring extends to the end of the original string.
The following code gets from the Niagara palindrome the substring that extends from index 11 up to, but not including, index 15, which is the word "roar":
String anotherPalindrome = "Niagara. O roar again!";
String roar = anotherPalindrome.su bstring(11, 15);

Other Methods for Manipulating Strings
Here are several other String methods for manipulating strings:
Other Methods in the String Class for Manipulating Strings
Method Description
String[] split(String regex)
String[] split(String regex, int limit) Searches for a match as specified by the string argument (which contains a regular expression) and splits this string into an array of strings accordingly. The optional integer argument specifies the maximum size of the returned array. Regular expressions are covered in the lesson titled "Regular Expressions."
CharSequence subSequence(int beginIndex, int endIndex) Returns a new character sequence constructed from beginIndex index up until endIndex - 1.
String trim() Returns a copy of this string with leading and trailing white space removed.
String toLowerCase()
String toUpperCase() Returns a copy of this string converted to lowercase or uppercase. If no conversions are necessary, these methods return the original string.
Searching for Characters and Substrings in a String
Here are some other String methods for finding characters or substrings within a string. The String class provides accessor methods that return the position within the string of a specific character or substring: indexOf() and lastIndexOf(). The indexOf() methods search forward from the beginning of the string, and the lastIndexOf() methods search backward from the end of the string. If a character or substring is not found, indexOf() and lastIndexOf() return -1.
The String class also provides a search method, contains, that returns true if the string contains a particular character sequence. Use this method when you only need to know that the string contains a character sequence, but the precise location isn't important.
The following table describes the various string search methods.
The Search Methods in the String Class
Method Description
int indexOf(int ch)
int lastIndexOf(int ch) Returns the index of the first (last) occurrence of the specified character.
int indexOf(int ch, int fromIndex)
int lastIndexOf(int ch, int fromIndex) Returns the index of the first (last) occurrence of the specified character, searching forward (backward) from the specified index.
int indexOf(String str)
int lastIndexOf(String str) Returns the index of the first (last) occurrence of the specified substring.
int indexOf(String str, int fromIndex)
int lastIndexOf(String str, int fromIndex) Returns the index of the first (last) occurrence of the specified substring, searching forward (backward) from the specified index.
boolean contains(CharSequenc e s) Returns true if the string contains the specified character sequence.
____________________ ____________________
Note: CharSequence is an interface that is implemented by the String class. Therefore, you can use a string as an argument for the contains() method.
____________________ ____________________
Replacing Characters and Substrings into a String
The String class has very few methods for inserting characters or substrings into a string. In general, they are not needed: You can create a new string by concatenation of substrings you have removed from a string with the substring that you want to insert.
The String class does have four methods for replacing found characters or substrings, however. They are:
Methods in the String Class for Manipulating Strings
Method Description
String replace(char oldChar, char newChar) Returns a new string resulting from replacing all occurrences of oldChar in this string with newChar.
String replace(CharSequence target, CharSequence replacement) Replaces each substring of this string that matches the literal target sequence with the specified literal replacement sequence.
String replaceAll(String regex, String replacement) Replaces each substring of this string that matches the given regular expression with the given replacement.
String replaceFirst(String regex, String replacement) Replaces the first substring of this string that matches the given regular expression with the given replacement.
An Example
The following class, Filename, illustrates the use of lastIndexOf() and substring() to isolate different parts of a file name.
____________________ ____________________
Note: The methods in the following Filename class don't do any error checking and assume that their argument contains a full directory path and a filename with an extension. If these methods were production code, they would verify that their arguments were properly constructed.
____________________ ____________________

public class Filename {
private String fullPath;
private char pathSeparator, extensionSeparator;

public Filename(String str, char sep, char ext) {
fullPath = str;
pathSeparator = sep;
extensionSeparator = ext;
}

public String extension() {
int dot = fullPath.lastIndexOf (extensionSeparator) ;
return fullPath.substring(d ot + 1);
}

public String filename() { // gets filename without extension
int dot = fullPath.lastIndexOf (extensionSeparator) ;
int sep = fullPath.lastIndexOf (pathSeparator);
return fullPath.substring(s ep + 1, dot);
}

public String path() {
int sep = fullPath.lastIndexOf (pathSeparator);
return fullPath.substring(0 , sep);
}
}
Here is a program, FilenameDemo, that constructs a Filename object and calls all of its methods:

public class FilenameDemo {
public static void main(String[] args) {
final String FPATH = "/home/mem/index.html";
Filename myHomePage = new Filename(FPATH,
'/', '.');
System.out.println("Extension = " +
myHomePage.extension ());
System.out.println("Filename = " +
myHomePage.filename( ));
System.out.println("Path = " +
myHomePage.path());
}
}
And here's the output from the program:
Extension = html
Filename = index
Path = /home/mem
As shown in the following figure, our extension method uses lastIndexOf to locate the last occurrence of the period (.) in the file name. Then substring uses the return value of lastIndexOf to extract the file name extension — that is, the substring from the period to the end of the string. This code assumes that the file name has a period in it; if the file name does not have a period, lastIndexOf returns -1, and the substring method throws a StringIndexOutOfBoun dsException.

Also, notice that the extension method uses dot + 1 as the argument to substring. If the period character (.) is the last character of the string, dot + 1 is equal to the length of the string, which is one larger than the largest index into the string (because indices start at 0). This is a legal argument to substring because that method accepts an index equal to, but not greater than, the length of the string and interprets it to mean "the end of the string."


Lesson: Packages
This lesson explains how to bundle classes and interfaces into packages, how to use classes that are in packages, and how to arrange your file system so that the compiler can find your source files.
Creating and Using Packages
To make types easier to find and use, to avoid naming conflicts, and to control access, programmers bundle groups of related types into packages.
____________________ ____________________
Definition: A package is a grouping of related types providing access protection and name space management. Note that types refers to classes, interfaces, enumerations, and annotation types. Enumerations and annotation types are special kinds of classes and interfaces, respectively, so types are often referred to in this lesson simply as classes and interfaces.
____________________ ____________________
The types that are part of the Java platform are members of various packages that bundle classes by function: fundamental classes are in java.lang, classes for reading and writing (input and output) are in java.io, and so on. You can put your types in packages too.
Suppose you write a group of classes that represent graphic objects, such as circles, rectangles, lines, and points. You also write an interface, Draggable, that classes implement if they can be dragged with the mouse.
//in the Draggable.java file
public interface Draggable {
. . .
}

//in the Graphic.java file
public abstract class Graphic {
. . .
}

//in the Circle.java file
public class Circle extends Graphic implements Draggable {
. . .
}

//in the Rectangle.java file
public class Rectangle extends Graphic implements Draggable {
. . .
}

//in the Point.java file
public class Point extends Graphic implements Draggable {
. . .
}

//in the Line.java file
public class Line extends Graphic implements Draggable {
. . .
}
You should bundle these classes and the interface in a package for several reasons, including the following:
• You and other programmers can easily determine that these types are related.
• You and other programmers know where to find types that can provide graphics-related functions.
• The names of your types won't conflict with the type names in other packages because the package creates a new namespace.
• You can allow types within the package to have unrestricted access to one another yet still restrict access for types outside the package.
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Default Re: Basics of starting java language


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yeh sabh aap nay khud likha hay kia :D
apko kya lagta ha...hehe
waise jo bhi smjh layn...
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