有个小问题请教

Java code

import java.awt.*;
import java.awt.event.*;

public class Test {
    public static void main(String[] args) {
        MFrame f=new MFrame("事件监听",100,100);
        Monitor m=new Monitor();
        Button b1=new Button("start");
        Button b2=new Button("stop");
        b1.addActionListener(m);
        b2.addActionListener(m);
        //b1.setActionCommand("f");
        //b2.setActionCommand("q");
        f.add(b1);
        f.add(b2);
    }

}
class MFrame extends Frame{
    public MFrame(String title,int width,int height){
        setLayout(new FlowLayout());
        setTitle(title);
        setSize(width,height);
        setLocation(100,100);
        setVisible(true);
    }
}
class Monitor implements ActionListener{
    public void actionPerformed(ActionEvent e) {
        //System.out.println("game will "+e.getActionCommand());
        Button b=(Button)e.getSource();
********        if(b==b1)System.out.println("wo");
        else System.out.println("ni");
        
    }
    
}

*******行出错，错在哪里？先谢谢各位了

------解决方案--------------------
if(b.getLabel().equals(b1.getLabel())
------解决方案--------------------
你的b1是局部变量，在monitor里面访问不到

------解决方案--------------------
你是在一个文件里定义了三个CLASS，这和你在三个文件里定义三个CLASS类似，Monitor这个类不可以直接引用Test里面的变量，你可以将二个BUTTON作为变量传入，代码如下
import java.awt.*;
import java.awt.event.*;

public class Test {
public static void main(String[] args) {
MFrame f=new MFrame("事件监听",100,100);
Button b1=new Button("start");
Button b2=new Button("stop");
Monitor m=new Monitor(b1,b2);
b1.addActionListener(m);
b2.addActionListener(m);
//b1.setActionCommand("f");
//b2.setActionCommand("q");
f.add(b1);
f.add(b2);
}
}

class MFrame extends Frame{
public MFrame(String title,int width,int height){
setLayout(new FlowLayout());
setTitle(title);
setSize(width,height);
setLocation(100,100);
setVisible(true);
}
}
class Monitor implements ActionListener{
Button b1=null;
Button b2=null;
public Monitor(Button b1,Button b2){
this.b1=b1;
this.b2=b2;
}
public void actionPerformed(ActionEvent e) {
//System.out.println("game will "+e.getActionCommand());
Button b=(Button)e.getSource();
if(b==b1)System.out.println("wo");
else System.out.println("ni");

}

}



------解决方案--------------------
if(b.getLabel().equals(b1.getLabel())

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