buttoncanvaspress.java

Java 入门书的源码

//Copyright (c) 1998, Arthur Gittleman
//This example is provided WITHOUT ANY WARRANTY either expressed or implied.

/* Adds two buttons and a canvas to an applet.
 * The canvas registers with each button to 
 * listen for button presses.  Its actionPerfomed
 * saves the button label, and asks the operating
 * system to redraw the canvas (calling paint).
 * Paint displays a message if Print was pressed
 * but clears the canvas if Clear was pressed./
 */

import java.awt.*;
import java.awt.event.*;
import java.applet.Applet;

public class ButtonCanvasPress extends Applet {
  private Button print = new Button("Print");
  private Button clear = new Button("Clear");
  private DrawOn canvas = new DrawOn();

  public void init() {
    add(print); 
    add(clear);
    print.addActionListener(canvas);
    clear.addActionListener(canvas);
    add(canvas);
    canvas.setBackground(Color.pink);
    canvas.setSize(150,150);
  }
}
class DrawOn extends Canvas implements ActionListener {
  String command = "";

  public void actionPerformed(ActionEvent event) {
    command = event.getActionCommand();
    repaint();
  } 
  public void paint(Graphics g) {
    if (command.equals("Print")){ 
      g.drawString("Hi there",20,20);
      g.drawString("You just pressed",20,40);
      g.drawString("the print button.",20,60);
    }
  }
}