编写一个应用程序有一个标题为计算的窗口窗口的布局为FlowLayout布局。设计四个按钮分别命名为加、差、积、、除；另外窗口中还有三个文本框。单击相应的按钮将两个文本框的数字做运算在第三个文本框中显示结果。

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;

public class Calculator extends JFrame implements ActionListener {
    private JLabel label1, label2, label3;
    private JTextField textField1, textField2, textField3;
    private JButton btnAdd, btnSub, btnMul, btnDiv;

    public Calculator() {
        setTitle("计算");
        setLayout(new FlowLayout());

        label1 = new JLabel("数字1：");
        add(label1);

        textField1 = new JTextField(10);
        add(textField1);

        label2 = new JLabel("数字2：");
        add(label2);

        textField2 = new JTextField(10);
        add(textField2);

        label3 = new JLabel("结果：");
        add(label3);

        textField3 = new JTextField(10);
        textField3.setEditable(false);
        add(textField3);

        btnAdd = new JButton("加");
        btnAdd.addActionListener(this);
        add(btnAdd);

        btnSub = new JButton("差");
        btnSub.addActionListener(this);
        add(btnSub);

        btnMul = new JButton("积");
        btnMul.addActionListener(this);
        add(btnMul);

        btnDiv = new JButton("除");
        btnDiv.addActionListener(this);
        add(btnDiv);

        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        setSize(300, 150);
        setVisible(true);
    }

    public void actionPerformed(ActionEvent e) {
        double num1 = Double.parseDouble(textField1.getText());
        double num2 = Double.parseDouble(textField2.getText());
        double result = 0;

        if (e.getSource() == btnAdd) {
            result = num1 + num2;
        } else if (e.getSource() == btnSub) {
            result = num1 - num2;
        } else if (e.getSource() == btnMul) {
            result = num1 * num2;
        } else if (e.getSource() == btnDiv) {
            result = num1 / num2;
        }

        textField3.setText(String.valueOf(result));
    }

    public static void main(String[] args) {
        new Calculator();
    }
}

运行程序后，效果如下图所示：

![Calculator.png](https://cdn.jsdelivr.net/gh/krislinzhao/IMGcloud/img/20200406143737.png