windowbuilder怎么写加减法计算器，请用代码实现

以下是一个简单的加减法计算器的代码，使用 WindowBuilder 开发：

import java.awt.EventQueue;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.border.EmptyBorder;
import javax.swing.JTextField;
import javax.swing.JButton;
import java.awt.event.ActionListener;
import java.awt.event.ActionEvent;

public class Calculator extends JFrame {

    private JPanel contentPane;
    private JTextField textField;
    private double num1, num2, result;
    private String operator;

    public static void main(String[] args) {
        EventQueue.invokeLater(new Runnable() {
            public void run() {
                try {
                    Calculator frame = new Calculator();
                    frame.setVisible(true);
                } catch (Exception e) {
                    e.printStackTrace();
                }
            }
        });
    }

    public Calculator() {
        setTitle("Calculator");
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        setBounds(100, 100, 250, 300);
        contentPane = new JPanel();
        contentPane.setBorder(new EmptyBorder(5, 5, 5, 5));
        setContentPane(contentPane);
        contentPane.setLayout(null);

        textField = new JTextField();
        textField.setBounds(10, 10, 214, 30);
        contentPane.add(textField);
        textField.setColumns(10);

        JButton btn1 = new JButton("1");
        btn1.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent e) {
                textField.setText(textField.getText() + "1");
            }
        });
        btn1.setBounds(10, 50, 50, 50);
        contentPane.add(btn1);

        JButton btn2 = new JButton("2");
        btn2.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent e) {
                textField.setText(textField.getText() + "2");
            }
        });
        btn2.setBounds(70, 50, 50, 50);
        contentPane.add(btn2);

        JButton btn3 = new JButton("3");
        btn3.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent e) {
                textField.setText(textField.getText() + "3");
            }
        });
        btn3.setBounds(130, 50, 50, 50);
        contentPane.add(btn3);

        JButton btn4 = new JButton("4");
        btn4.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent e) {
                textField.setText(textField.getText() + "4");
            }
        });
        btn4.setBounds(10, 110, 50, 50);
        contentPane.add(btn4);

        JButton btn5 = new JButton("5");
        btn5.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent e) {
                textField.setText(textField.getText() + "5");
            }
        });
        btn5.setBounds(70, 110, 50, 50);
        contentPane.add(btn5);

        JButton btn6 = new JButton("6");
        btn6.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent e) {
                textField.setText(textField.getText() + "6");
            }
        });
        btn6.setBounds(130, 110, 50, 50);
        contentPane.add(btn6);

        JButton btn7 = new JButton("7");
        btn7.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent e) {
                textField.setText(textField.getText() + "7");
            }
        });
        btn7.setBounds(10, 170, 50, 50);
        contentPane.add(btn7);

        JButton btn8 = new JButton("8");
        btn8.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent e) {
                textField.setText(textField.getText() + "8");
            }
        });
        btn8.setBounds(70, 170, 50, 50);
        contentPane.add(btn8);

        JButton btn9 = new JButton("9");
        btn9.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent e) {
                textField.setText(textField.getText() + "9");
            }
        });
        btn9.setBounds(130, 170, 50, 50);
        contentPane.add(btn9);

        JButton btn0 = new JButton("0");
        btn0.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent e) {
                textField.setText(textField.getText() + "0");
            }
        });
        btn0.setBounds(70, 230, 50, 50);
        contentPane.add(btn0);

        JButton btnPlus = new JButton("+");
        btnPlus.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent e) {
                num1 = Double.parseDouble(textField.getText());
                operator = "+";
                textField.setText("");
            }
        });
        btnPlus.setBounds(190, 50, 50, 50);
        contentPane.add(btnPlus);

        JButton btnMinus = new JButton("-");
        btnMinus.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent e) {
                num1 = Double.parseDouble(textField.getText());
                operator = "-";
                textField.setText("");
            }
        });
        btnMinus.setBounds(190, 110, 50, 50);
        contentPane.add(btnMinus);

        JButton btnEquals = new JButton("=");
        btnEquals.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent e) {
                num2 = Double.parseDouble(textField.getText());
                if (operator.equals("+")) {
                    result = num1 + num2;
                } else if (operator.equals("-")) {
                    result = num1 - num2;
                }
                textField.setText(Double.toString(result));
            }
        });
        btnEquals.setBounds(190, 170, 50, 110);
        contentPane.add(btnEquals);
    }
}