java计算器程序，只能进行四则运算

package a;

import java.awt.Button;
import java.awt.GridLayout;
import java.awt.Panel;
import java.awt.TextField;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

import javax.swing.JFrame;
import javax.swing.JLabel;

public class Calculator extends JFrame implements ActionListener {

private static final long serialVersionUID = 1L;
// public static final int EXIT_ON_CLOSE = 3;
 
 private Button plus=new Button("+");  //运算符
 private Button minus=new Button("-");
 private Button multip=new Button("*");
 private Button divise=new Button("/");
 
 private Panel pan=new Panel();
 private JLabel opl1 = new JLabel("操作数1");
 private JLabel opl2 = new JLabel("操作数2");
 private JLabel resl = new JLabel("结果：");
 
 private TextField op1=new TextField(10);
 private TextField op2=new TextField(10);
 private TextField result=new TextField(10);