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The Life Cycle of an Object

A typical Java program creates many objects, which interact with one another by sending each other messages. Through these object interactions, a Java program can implement a GUI, run an animation, or send and receive information over a network. Once an object has completed the work for which it was created, it is garbage-collected and 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) {

        // create a point object and two rectangle objects
        Point origin_one = new Point(23, 94);
        Rectangle rect_one = new Rectangle(origin_one, 100, 200);
        Rectangle rect_two = new Rectangle(50, 100);

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

        // set rect_two's position
        rect_two.origin = origin_one;

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

        // move rect_two and display its new position
        rect_two.move(40, 72);
        System.out.println("X Position of rect_two: " + rect_two.origin.x);
        System.out.println("Y Position of rect_two: " + rect_two.origin.y);
    }
}

After creating the objects, the program manipulates the objects and displays some information about them. Here's the output from the program:

Width of rect_one: 100
Height of rect_one: 200
Area of rect_one: 20000
X Position of rect_two: 23
Y Position of rect_two: 94
X Position of rect_two: 40
Y Position of rect_two: 72

This section uses this example to describe the life cycle of an object within a program. From this, you can learn how to write code that creates and uses an object and how the system cleans it up.

• Creating Objects
• Using Objects
• Cleaning Up Unused Objects
• Summary of Creating and Using Objects
• Questions and Exercises: Creating and Using Objects

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