Java Applet基础

事件处理

Applet类从Container类继承了许多事件处理方法。Container类定义了几个方法，例如：processKeyEvent()和processMouseEvent()，用来处理特别类型的事件，还有一个捕获所有事件的方法叫做processEvent。

为了响应一个事件，applet必须重写合适的事件处理方法。

import java.awt.event.MouseListener;
import java.awt.event.MouseEvent;
import java.applet.Applet;
import java.awt.Graphics;
 
public class ExampleEventHandling extends Applet
                             implements MouseListener {
 
    StringBuffer strBuffer;
 
    public void init() {
         addMouseListener(this);
         strBuffer = new StringBuffer();
        addItem("initializing the apple ");
    }
 
    public void start() {
        addItem("starting the applet ");
    }
 
    public void stop() {
        addItem("stopping the applet ");
    }
 
    public void destroy() {
        addItem("unloading the applet");
    }
 
    void addItem(String word) {
        System.out.println(word);
        strBuffer.append(word);
        repaint();
    }
 
    public void paint(Graphics g) {
         //Draw a Rectangle around the applet's display area.
        g.drawRect(0, 0,
                      getWidth() - 1,
                      getHeight() - 1);
 
         //display the string inside the rectangle.
        g.drawString(strBuffer.toString(), 10, 20);
    }
 
  
    public void mouseEntered(MouseEvent event) {
    }
    public void mouseExited(MouseEvent event) {
    }
    public void mousePressed(MouseEvent event) {
    }
    public void mouseReleased(MouseEvent event) {
    }
 
    public void mouseClicked(MouseEvent event) {
         addItem("mouse clicked! ");
    }
}

如下调用该applet：

<html>
<title>Event Handling</title>
<hr>
<applet code="ExampleEventHandling.class" width="300" height="300">
</applet>
<hr>
</html>

最开始运行，applet显示 "initializing the applet. Starting the applet."，然后你一点击矩形框，就会显示"mouse clicked" 。

显示图片

applet能显示GIF,JPEG,BMP等其他格式的图片。为了在applet中显示图片，你需要使用java.awt.Graphics类的drawImage()方法。

如下实例演示了显示图片的所有步骤：

import java.applet.*;
import java.awt.*;
import java.net.*;
public class ImageDemo extends Applet
{
  private Image image;
  private AppletContext context;
  public void init()
  {
      context = this.getAppletContext();
      String imageURL = this.getParameter("image");
      if(imageURL == null)
      {
         imageURL = "java.jpg";
      }
      try
      {
         URL url = new URL(this.getDocumentBase(), imageURL);
         image = context.getImage(url);
      }catch(MalformedURLException e)
      {
         e.printStackTrace();
         // Display in browser status bar
         context.showStatus("Could not load image!");
      }
   }
   public void paint(Graphics g)
   {
      context.showStatus("Displaying image");
      g.drawImage(image, 0, 0, 200, 84, null);
      g.drawString("www.javalicense.com", 35, 100);
   } 
}

如下调用该applet：

<html>
<title>The ImageDemo applet</title>
<hr>
<applet code="ImageDemo.class" width="300" height="200">
<param name="image" value="java.jpg">
</applet>
<hr>
</html>