package com.yxk.test;

/**
 * ClassName:MyFrame
 * @Description: TODO(这里用一句话描述这个类的作用) 
 * @author   yxk
 * @version 1.0 2011-9-13
 * @since 1.0
 * @see
 */
import java.awt.*;
import java.awt.event.*;

public class MyFrame extends Frame implements ActionListener{
    /**
     * @Description: TODO(添加属性注释)
     */
    private static final long serialVersionUID = 1L;
    public FlowLayout f = new FlowLayout();
    public TextField tf1 = new TextField(10);
    public TextField tf2 = new TextField(10);
    public TextField tf3 = new TextField(15);
    public Label l1 = new Label("+");
    public Button b = new Button("=");

    public MyFrame() {
        this.addWindowListener(new WindowAdapter() {
            @Override
            public void windowClosing(WindowEvent e) {
                System.exit(0);
            }
        });

    }

    public void launch() {

        setLayout(f);
        add(tf1);
        add(l1);
        add(tf2);
        add(b);
        add(tf3);
        b.addActionListener(this);
        pack();
        setVisible(true);
    }

    @Override
    public void actionPerformed(ActionEvent e) {
        
        int num1 = Integer.parseInt(tf1.getText());
        int num2 = Integer.parseInt(tf2.getText());
        tf3.setText(" " + (num1 + num2));
        
    }

    public static void main(String[] args) {
        new MyFrame().launch();
    }

}

------解决方案--------------------
错误是不是找不到变量，你那几个tf1,tf2,tf3,在方法里面声明的！对像是找不到的！
   你把他改成成员变量，然后在方法里面实例化；但注意的是，这些成员变量一定要在点用前实例化，否着会报nullclassexception