package PKG;

import javax.swing.*;

import java.awt.event.*;
/**
 *
 * @author chengming
 */
public class TestAdd extends JFrame{
    public static JTextField tf1,tf2,tf3;
    public static void main(String args[])
    {
        JFrame jf=new JFrame("小明的窗口");
        jf.setSize(600,600);
        
        tf1=new JTextField(10);
        JLabel jb1=new JLabel("+");
        tf2=new JTextField(10);
        JButton b=new JButton("=");
        b.addActionListener(new A());
       tf3=new JTextField(10);
        
        JPanel jp=new JPanel();
        jp.add(tf1);
        jp.add(jb1);
        jp.add(tf2);
        jp.add(b);
        jp.add(tf3);
        jf.add(jp);        
        
        jf.setVisible(true);
        jf.pack();
        
         }
}


 class A implements ActionListener
{
  public  void ActionPerformed(ActionEvent e)
    {
        String str1=TestAdd.tf1.getText();
        String str2=TestAdd.tf2.getText();
        int i1=Integer.parseInt(str1);
        int i2=Integer.parseInt(str2);
        int i3=i1+i2;
        TestAdd.tf3.setText(i3+"");
        
    }

TestAdd 都已经继承 JFrame 了，干嘛还生成一个JFrame 你到底想搞出几个JFrame啊。

还有ActionPerformed 写作了，A小写

import javax.swing.*;
import java.awt.event.*;
/**
*
* @author chengming
*/
public class TestAdd extends JFrame{
    public MyJPanel jp=null;
   
    public TestAdd(String s){
        super(s);
        jp=new MyJPanel();
        this.add(jp);       
       
        this.setSize(600,600);
        this.pack();
        this.setDefaultCloseOperation(EXIT_ON_CLOSE);
        this.setVisible(true);
    }
    public static void main(String args[])
    {
        TestAdd t=new TestAdd("小明的窗口");
       
    }
}


class MyJPanel extends JPanel implements ActionListener{
    public  JTextField tf1,tf2,tf3;
    public MyJPanel(){
        tf1=new JTextField(10);
        JLabel jb1=new JLabel("+");
        tf2=new JTextField(10);
        JButton b=new JButton("=");
        tf3=new JTextField(10);
        b.addActionListener(this);
        this.add(tf1);
        this.add(jb1);
        this.add(tf2);
        this.add(b);
        this.add(tf3);
    }
  public  void actionPerformed(ActionEvent e)
    {
        String str1=tf1.getText();
        String str2=tf2.getText();
        int i1=Integer.parseInt(str1);
        int i2=Integer.parseInt(str2);
        int i3=i1+i2;
        tf3.setText(i3+"");
       
    }

}

谢了