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Java in a Nutshell

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7.5 Scribbling with Inner Classes

The Java 1.1 event model was designed to work well with another new Java 1.1 feature: inner classes. Example 7.3 shows what the applet looks like when the event listeners are implemented as anonymous inner classes. Note how succinct this representation is. This is perhaps the most common way to use the Java 1.1 event model, so you'll probably see a lot of code that looks like this. In this case, our simple applet is nothing but event-handling code, so this version of it consists almost entirely of anonymous class definitions.

Note that we've added a feature to the applet. It now includes a Clear button. An ActionListener object is registered with the button; it clears the scribble when the appropriate event occurs.

Example 7.3: Scribble: Using Inner Classes

import java.applet.*;
import java.awt.*;
import java.awt.event.*;
public class Scribble3 extends Applet {
  int last_x, last_y;
  public void init() {
    // Define, instantiate, and register a MouseListener object.
    this.addMouseListener(new MouseAdapter() {
      public void mousePressed(MouseEvent e) {
        last_x = e.getX();
        last_y = e.getY();
      }
    });
    // Define, instantiate, and register a MouseMotionListener object.
    this.addMouseMotionListener(new MouseMotionAdapter() {
      public void mouseDragged(MouseEvent e) {
        Graphics g = getGraphics();
        int x = e.getX(), y = e.getY();
        g.setColor(Color.black);
        g.drawLine(last_x, last_y, x, y);
        last_x = x; last_y = y;
      }
    });
    // Create a clear button.
    Button b = new Button("Clear");
    // Define, instantiate, and register a listener to handle button presses.
    b.addActionListener(new ActionListener() {
      public void actionPerformed(ActionEvent e) {  // clear the scribble
        Graphics g = getGraphics();
        g.setColor(getBackground());
        g.fillRect(0, 0, getSize().width, getSize().height);
      }
    });
    // And add the button to the applet.
    this.add(b);
  }
}


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