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).

Source - http://java.sun.com/docs/books/tutorial/java/javaOO/classes.html

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.

Source - http://java.sun.com/docs/books/tutorial/java/javaOO/objects.html