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

import javax.swing.*;
public class Test {
    public static void main(String[] args) {
        EventQueue.invokeLater(new Runnable(){
            public void run(){
                MyFrame frame=new MyFrame();                
                frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                
            }
        });
    }

}
class MyFrame extends JFrame{
    public MyFrame(){
    Toolkit kit=Toolkit.getDefaultToolkit();
    Dimension screenSize=kit.getScreenSize();
    setSize(screenSize.width/2,screenSize.height/2);
    setLocation(100,100);
    setVisible(true);
    TextField tf1=new TextField(10);
    TextField tf2=new TextField(10);
    TextField tf3=new TextField(20);
    Label equal=new Label("+");
    Panel p=new Panel();
    Button b=new Button("=");
    p.add(tf1);
    p.add(equal);
    p.add(tf2);
    p.add(b);
    p.add(tf3);                
    add(p);
    Monitor m=new Monitor(this);
    b.addActionListener(m);
    pack();
    
    }
}
class Monitor implements ActionListener{
    MyFrame mf=null;
    public Monitor(MyFrame mf){
        this.mf=mf;
    }
    public void actionPerformed(ActionEvent e) {
*        double a=Double.parseDouble(mf.tf1.getText());
*        double bb=Double.parseDouble(mf.tf2.getText());
*        mf.tf3.setText(""+(a+bb));
    }
    
}

class MyFrame extends JFrame{
    public MyFrame(){
    Toolkit kit=Toolkit.getDefaultToolkit();
    Dimension screenSize=kit.getScreenSize();
    setSize(screenSize.width/2,screenSize.height/2);
    setLocation(100,100);
    setVisible(true);
    //TextField tf1=new TextField(10); //这样定义的是局部变量，方法内有效，方法外无法访问
    //TextField tf2=new TextField(10);
    //TextField tf3=new TextField(20);
    Label equal=new Label("+");
    Panel p=new Panel();
    Button b=new Button("=");
    p.add(tf1);
    p.add(equal);
    p.add(tf2);
    p.add(b);
    p.add(tf3);                
    add(p);
    Monitor m=new Monitor(this);
    b.addActionListener(m);
    pack();
    
    }
    TextField tf1=new TextField(10); //这样定义的是成员变量，这里没有public修饰
     TextField tf2=new TextField(10); //同一个包的外部类可以访问
    TextField tf3=new TextField(20);

}