import javax.swing.*;
import java.awt.*;
import java.awt.event.*;

class Tim extends JFrame{
JPanel pnl;
JButton btn1=new JButton("开始");
JButton btn2=new JButton("停止");
FlowLayout flowLayout=new FlowLayout();
Timer obj;
Class cl;

Tim(){
super("Timer");
men();
action();
}
void men(){
pnl=(JPanel)getContentPane();
pnl.setLayout(flowLayout);
setSize(new Dimension(400,300));
pnl.add(btn1);
pnl.add(btn2);
this.setVisible(true);
}
void action(){
btn1.addActionListener(new ActionListener(){
public void actionPerformed(ActionEvent e){
obj=new Timer(5000,this);
obj.start();
System.out.println("aaaaaaaaaaaaaaaa");
}
});
btn2.addActionListener(new ActionListener(){
public void actionPerformed(ActionEvent e){
obj.stop();//这里的obj为null，为什么我在上面属性区创建了引用，在btn1里面声明，影响不到btn2里面来呢？这个问题如何解决啊？
}
});
}

public static void main(String[] args){
Tim cc=new Tim();
}
}

试试把obj放到action 这个大方法下面，声明为final的　作用域的问题

[CODE]
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;

class Tim extends JFrame{
JPanel pnl;
JButton btn1=new JButton("开始");
JButton btn2=new JButton("停止");
FlowLayout flowLayout=new FlowLayout();

Class cl;
ActionListener listener=new ActionListener(){
public void actionPerformed(ActionEvent e){

obj.start();
System.out.println("aaaaaaaaaaaaaaaa");
}
};
Timer obj=new Timer(500,listener);

Tim(){
super("Timer");
men();
action();
}
void men(){
pnl=(JPanel)getContentPane();
pnl.setLayout(flowLayout);
setSize(new Dimension(400,300));
pnl.add(btn1);
pnl.add(btn2);
this.setVisible(true);
}
void action(){
btn1.addActionListener(listener);
btn2.addActionListener(new ActionListener(){
public void actionPerformed(ActionEvent e){
obj.stop();
}
});
}

public static void main(String[] args){
Tim cc=new Tim();
}
}

[/CODE]

内部类。

类部类是这样写的么？。。。。。。
看到要再去看看了