代码：

package ch10;
import javax.swing.*;
import java.awt.*;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

public class Demo02 extends JFrame implements ActionListener {
	JTextArea display;
   JButton[] buttons;
   String[] buttonLabels = {
            "1", "2", "3", "+","C", 
            "4", "5", "6", "-","退格", 
            "7", "8", "9", "*","1/x", 
            "0", "+/-", ".", "/","="};
    String input = "";
    double num1, num2;
    char operator;

    public Demo02() {
        setTitle("计算器");
        setSize(400, 400);
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        setLayout(new BorderLayout());

        display = new JTextArea();
        display.setFont(new Font("Serif", Font.PLAIN, 50));
        add(display, BorderLayout.NORTH);

        JPanel buttonPanel = new JPanel(new GridLayout(4,5,2,2));
        buttons = new JButton[20];
        for (int i = 0; i < 20; i++) {
            buttons[i] = new JButton(buttonLabels[i]);
            buttons[i].setFont(new Font("Serif", Font.BOLD, 18));
            buttons[i].setForeground(Color.BLUE);
            buttons[i].addActionListener(this);
            buttonPanel.add(buttons[i]);
        }
        buttons[4].setForeground(Color.RED);
        buttons[9].setForeground(Color.RED);
        buttons[19].setForeground(Color.RED);
        add(buttonPanel, BorderLayout.CENTER);

        
    }

    public void actionPerformed(ActionEvent e) {
        String action = e.getActionCommand();
        if (action.matches("[0-9]")) {
            input += action;
            display.setText(input);
        
        } else if (action.equals("C")) {
        	input = input.substring(0, 0);
        	display.setText(input);
        }else if (action.equals("退格")) {
              if (input.length() > 0) {
                   input = input.substring(0, input.length() - 1);
                   display.setText(input);
                     }
               
        } else if (action.matches("[+\\-*/]")) {
            num1 = Double.parseDouble(input);
            operator = action.charAt(0);
            input = "";
        } else if (action.equals(".")) {
            if (!input.contains(".")) {
                input += ".";
                display.setText(input);
            }
        } else if (action.equals("=")) {
            num2 = Double.parseDouble(input);
            double result = 0;
            switch (operator) {
                case '+':
                    result = num1 + num2;
                    break;
                case '-':
                    result = num1 - num2;
                    break;
                case '*':
                    result = num1 * num2;
                    break;
                case '/':
                    result = num1 / num2;
                    break;
            }
            display.setText(String.valueOf(result));
            input = "";
        }
    }

    public static void main(String[] args) {
		Demo02 demo=new Demo02();
		demo.setVisible(true);
	}
}

运行结果：