Java监听器用法（三）：外部类监听器

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
public class ButtonFrame extends JFrame
{ //继承框架
   private JPanel buttonPanel;
   private static final int DEFAULT_WIDTH = 300;
   private static final int DEFAULT_HEIGHT = 200;
   ColorListener yellowListener,blueListener, redListener;//声明监听器，作为一个成员变量
   public ButtonFrame()
   {      
      setSize(DEFAULT_WIDTH, DEFAULT_HEIGHT);
      // 创建按钮对象
      JButton yellowButton = new JButton("Yellow");
      JButton blueButton   = new JButton("Blue");
      JButton redButton    = new JButton("Red");
      buttonPanel = new JPanel();
      // 添加按钮到画板
      buttonPanel.add(yellowButton);
      buttonPanel.add(blueButton);
      buttonPanel.add(redButton);    
      add(buttonPanel);
      // 创建按钮监听器，要传入组件的引用
     yellowListener = new ColorListener(Color.YELLOW, buttonPanel);
     blueListener   = new ColorListener(Color.BLUE, buttonPanel);
    redListener    = new ColorListener(Color.RED, buttonPanel);
      // 为每个按钮设置监听器
      yellowButton.addActionListener(yellowListener);
      blueButton.addActionListener(blueListener);
      redButton.addActionListener(redListener);
   }   
}

//监听器单独作为一个外部类
class ColorListener implements ActionListener  
{//实现监听接口
   private Color backgroundColor;
   private JPanel panel;
   public ColorListener(Color c,JPanel p) {//单独作为一个类时要传入要修改组件的引用
      backgroundColor = c;
      panel=p;
   }
   public void actionPerformed(ActionEvent event) {
      panel.setBackground(backgroundColor);//要把引用传给监听器类
   }
}