JAVA编程计数器类Counter

import java.applet.*;
import java.awt.*;
import java.awt.event.*;
public class counter extends Applet implements ActionListener

{    int a=0;
Label Lbl ;
TextField input;
Button btn1 ;
Button btn2;
Button btn3 ;
public void init()

{   Lbl = new Label("请输入数字：    ");
    input = new TextField(6);
    btn1 = new Button("计数器加一");
    btn2 = new Button("计数器减一");
    btn3 = new Button("计数器清零");
add(Lbl);
    add(input);
add(btn1);
add(btn2);
add(btn3);

input.addActionListener(this);
btn1.addActionListener(this);
btn2.addActionListener(this);
btn3.addActionListener(this);