import java.awt.*;
import java.awt.event.*;
import java.applet.*;
import javax.swing.*;
import javas.swing.border.*;
class MyButton extends Button implements ActionListener
{
MyButton()
{
TextField tf1=new TextField();
TextField tf2=new TextField();
TextField tf3=new TextField();
add(tf1);
Box box1;
box1=Box.createVericalBox();
Button b1=new Button("+");
Button b2=new Button("-");
Button b3=new Button("*");
Button b4=new Button("/");
box1.add(b1);
box1.add(b2);
box1.add(b3);
box1.add(b4);
add(box1);
add(tf2);
add(tf3);
b1.addActionListener(this);
b2.addActionListener(this);
b3.addActionListener(this);
b4.addActionListener(this);
}
public void actionPerformed(ActionEvent e)
{
int a=Integer.parseInt(tf1.getText());
int b=Integer.parseInt(tf2.getText());
if(e.getSource()==b1)
{
tf3.setText(a+b);
}
else if(e.getSource()==b2)
{
tf3.setText(a-b);
}
else if(e.getSource()==b3)
{
tf3.setText(a*b);
}
else uf(e.getSource==b4)
{
tf3.setText(a/b);
}
}
}
class aextends Applet
{
MyButton b;
public void init()
{
b=new MyButton();
}
}

我不太理解, 你为什么将这个class 定义为Button的继承类,
尽量保留你的代码, 修改了一下.

import java.awt.*;
import java.awt.event.*;
import java.applet.*;
import javax.swing.*;
import javax.swing.Box;
import javax.swing.JFormattedTextField;
import java.text.NumberFormat;
import javax.swing.border.*;
class MyButton extends JButton implements ActionListener
{
  JFormattedTextField tf1 = new JFormattedTextField(NumberFormat.getIntegerInstance());
  JFormattedTextField tf2 = new JFormattedTextField(NumberFormat.getIntegerInstance());
  
  JTextField tf3 = new JTextField();

  Button b1 = new Button(\"+\");
  Button b2 = new Button(\"-\");
  Button b3 = new Button(\"*\");
  Button b4 = new Button(\"/\");

  Box box1 = Box.createVerticalBox();
  Box box2 = Box.createHorizontalBox();

  
  MyButton()
  {
    box1.add(b1);
    box1.add(b2);
    box1.add(b3);
    box1.add(b4);
    tf1.setValue(new Integer(1));
    tf2.setValue(new Integer(1));
    box2.add(tf1);
    box2.add(box1);
    box2.add(tf2);
    box2.add(tf3);
    tf3.setEditable(false);
    add(box2);
    b1.addActionListener(this);
    b2.addActionListener(this);
    b3.addActionListener(this);
    b4.addActionListener(this);
    
  }
   
  public void actionPerformed(ActionEvent e)
  {
    int a = Integer.parseInt(tf1.getText());
    int b = Integer.parseInt(tf2.getText());
    double result = 0;
    if(e.getSource()==b1)
    {
      result = a + b;
    }
    else if(e.getSource()==b2)
    {
      result = a - b;
    }
    else if(e.getSource()==b3)
    {
      result = a * b;
    }
    else if(e.getSource()==b4)
    {
      result = (double) a / (double) b;
    }
    tf3.setText(new Double(result).toString());
  }
}

public class AppletTest extends Applet
{
  public void init()
  {
    MyButton b = new MyButton();
    b.setPreferredSize(new java.awt.Dimension(400, 300));
    add(b);
  }
}

程序的结构错误：
import java.awt.*;
import java.awt.event.*;
import java.applet.*;
import javax.swing.*;

public class Test extends Applet implements ActionListener
{
TextField tf1;
TextField tf2;
TextField tf3;
Box box1;
Button b1;
Button b2;
Button b3;
Button b4;

public void init()
{
tf1=new TextField();
tf2=new TextField();
tf3=new TextField();
add(tf1);
box1=Box.createVerticalBox();
b1=new Button("+");
b2=new Button("-");
b3=new Button("*");
b4=new Button("/");
box1.add(b1);
box1.add(b2);
box1.add(b3);
box1.add(b4);
add(box1);
add(tf2);
add(tf3);
b1.addActionListener(this);
b2.addActionListener(this);
b3.addActionListener(this);
b4.addActionListener(this);
}
public void start(){}
public void stop(){}
public void destory(){}
public void actionPerformed(ActionEvent e)
{
int a=Integer.parseInt(tf1.getText());
int b=Integer.parseInt(tf2.getText());
if(e.getSource()==b1)
{
tf3.setText(new Integer(a+b).toString());
}
else if(e.getSource()==b2)
{
tf3.setText(new Integer(a-b).toString());
}
else if(e.getSource()==b3)
{
tf3.setText(new Integer(a*b).toString());
}
else if(e.getSource()==b4)
{
tf3.setText(new Integer(a/b).toString());
}
}
}

谢谢,再问一下,程序的结构有什么要求??