Extending the AWT in Java

ou can add more elements to your interface toolbox in two ways: you can extend existing components yourself or you can use someone else's classes.

Extending Components (the Image Button)

One of the nice features of object-oriented programming is that you can extend existing classes and add new functionality with minimal effort. The applet in Listing 17.43 contains the code for a class called image_button_component, which behaves like a button but has an image for its interface rather than the regular button picture.

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Listing 17.43. The image_button_component class.

import java.applet.Applet;

import java.awt.*;

public class image_button extends Applet {

    MediaTracker the_tracker;

    image_button_component  the_button; 

    public void init()

    {

        Image the_picture, inverse_picture;

        //load the two images, one for the regular 

        //button image and one for when the

        //mouse button is held down on the button

        the_tracker = new MediaTracker(this);

        the_picture = getImage(getDocumentBase(),"test.gif");

        the_tracker.addImage(the_picture,0); 

        inverse_picture = getImage(getDocumentBase(),"invert.gif");

        the_tracker.addImage(inverse_picture,0); 

        //Wait until both images are fully loaded.

        try {

            the_tracker.waitForID(0); 

        } catch(InterruptedException e) {}

        the_button = new image_button_component(the_picture,inverse_picture);

        add(the_button); 

    }

    public boolean mouseDown(Event e, int x, int y) {

        if (e.target instanceof image_button_component) {

            System.out.println("image_button_component clicked");

        }

        return true;

    }

}

class image_button_component extends Canvas {

    Image my_image, inverse_image;

    boolean mousedown_flag;

    public image_button_component(Image an_image,Image i_image) {

        my_image = an_image; 

        inverse_image = i_image;

        //the button starts unclicked

        mousedown_flag = false;

        //make sure the canvas image is drawn

        repaint();

    }

    //override these methods to tell the layout manager 

    //the size of the button which is defined by the

    //size of the image. This assumes that the two images are the same size.

    public Dimension minimumSize() {

        return new Dimension(my_image.getWidth(null),my_image.getHeight(null)); 

    }

    public Dimension preferredSize() {

        return new Dimension(my_image.getWidth(null),my_image.getHeight(null)); 

    }

    //make sure the button image is changed when the mouse is clicked

    public boolean mouseDown(Event e, int x, int y) {

        mousedown_flag = true;

        repaint();

        //return false so the event is passed on so the mouseDown 

        //method in the applet can use it

        return false; 

    }

    public boolean mouseUp(Event e, int x, int y) {

        mousedown_flag = false;

        repaint();

        //return true because the applet doesn't need this 

        //event but you might want to

        //return false to make this more button like

        return true;

    }

    public void paint(Graphics g) {

        //determine which image to draw based on the mouse state

        if (!mousedown_flag) {

            g.drawImage(my_image,0,0,null); 

        } else {

            g.drawImage(inverse_image,0,0,null); 

        }

    }

}

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The applet loads in two images-one for the regular button and one for when it's clicked-and then creates a new instance of the image_button_component class.