Java事件处理 P120 例6-2（按钮事件处理程序）

源程序ButtonDemo.java是按钮事件处理程序。

import java.awt.*;import java.awt.event.*;import java.applet.*;public class ButtonDemo extends Applet implements ActionListener  {     String msg=" ";     public void init()     {         Button yes = new Button("是"); Button no = new Button("否");  Button maybe = new Button("取消"); add(yes); add(no); add(maybe); yes.addActionListener(this); no.addActionListener(this); maybe.addActionListener(this);  }     public void actionPerformed(ActionEvent e)         {          String str = e.getActionCommand();  if(str=="是")  {   msg="您单击了按钮'是'.";          }  else if (str=="否")          {   msg="您单击了按钮'否'.";   }  else  {  msg="您单击了按钮‘取消’.";  }  repaint();  }  public void paint(Graphics g)  {  g.drawString(msg,6,100);  }   }