一个java细节。

要把实现了某种接口的类的对象注册到事件源对象上

eg1：

import java.awt.*;
 import java.awt.event.*;
 public class A implements  ActionListener{
     public static void main(String args[]) {
Frame f = new Frame("Test");
Button b1 = new Button("Start");
Button b2 = new Button("Stop");b1.addActionListener(this);       
b2.addActionListener(this);
b2.setActionCommand("game over");
f.add(b1,"North");       
f.add(b2,"Center");
f.pack();        
f.setVisible(true);
     }
     public void actionPerformed(ActionEvent e) {
     System.out.println("a button has been pressed," + 
     "the relative info is:\n " + e.getActionCommand());    
     }
 }

---------会报错----------------------------------------------------

正确的应该是：eg2：

import java.awt.*;
 import java.awt.event.*;
 public class A implements  ActionListener{
     public static void main(String args[]) {
Frame f = new Frame("Test");
Button b1 = new Button("Start");
Button b2 = new Button("Stop");
A aa=new A();

b1.addActionListener(aa);       
b2.addActionListener(aa);
b2.setActionCommand("game over");
f.add(b1,"North");       
f.add(b2,"Center");
f.pack();        
f.setVisible(true);
     }
     public void actionPerformed(ActionEvent e) {
     System.out.println("a button has been pressed," + 
     "the relative info is:\n " + e.getActionCommand());    
     }
 }