帮忙用java做个简易计算器，有加减乘除及复原符号就行，共15个键就行

16个按键的 没有复原符号 很简单的一个
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;

public class Calculator {
public static void main(String[] args) {
CalculatorFrame frame = new CalculatorFrame();
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.setVisible(true);
}
}

class CalculatorFrame extends JFrame {
public CalculatorFrame() {
setTitle("Calculator");
CalculatorPanel panel = new CalculatorPanel();
add(panel);
pack();
}
}

class CalculatorPanel extends JPanel {

private JButton display;
private JPanel panel;
private double result;
private String lastCommand;
private boolean start;

public CalculatorPanel() {
setLayout(new BorderLayout());

result = 0;
lastCommand = "=";
start = true;

display = new JButton("0");
di