修改程序Example11_13.java,要求按照MVC开发模式来编写，设计一个MathListener.java,实现对ActionEvent事件的监听，不再使用窗体做监视器对象。

好的，根据您的要求，我对Example11_13.java进行了修改，按照MVC开发模式来编写，设计了一个MathListener.java来实现对ActionEvent事件的监听，不再使用窗体做监视器对象。修改后的代码如下：

MathModel.java

public class MathModel {
    private int num1;
    private int num2;
    private int result;

    public void setNum1(int num1) {
        this.num1 = num1;
    }

    public void setNum2(int num2) {
        this.num2 = num2;
    }

    public int getResult() {
        return result;
    }

    public void add() {
        result = num1 + num2;
    }

    public void subtract() {
        result = num1 - num2;
    }

    public void multiply() {
        result = num1 * num2;
    }

    public void divide() {
        result = num1 / num2;
    }
}

MathView.java

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

public class MathView extends JFrame {
    private JLabel num1Label;
    private JLabel num2Label;
    private JLabel resultLabel;
    private JTextField num1TextField;
    private JTextField num2TextField;
    private JTextField resultTextField;
    private JButton addButton;
    private JButton subtractButton;
    private JButton multiplyButton;
    private JButton divideButton;

    public MathView() {
        setTitle("Calculator");
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        setLayout(new GridLayout(4, 3));

        num1Label = new JLabel("Num1:");
        add(num1Label);

        num1TextField = new JTextField();
        add(num1TextField);

        num2Label = new JLabel("Num2:");
        add(num2Label);

        num2TextField = new JTextField();
        add(num2TextField);

        resultLabel = new JLabel("Result:");
        add(resultLabel);

        resultTextField = new JTextField();
        resultTextField.setEditable(false);
        add(resultTextField);

        addButton = new JButton("+");
        add(addButton);

        subtractButton = new JButton("-");
        add(subtractButton);

        multiplyButton = new JButton("*");
        add(multiplyButton);

        divideButton = new JButton("/");
        add(divideButton);

        pack();
        setVisible(true);
    }

    public int getNum1() {
        return Integer.parseInt(num1TextField.getText());
    }

    public int getNum2() {
        return Integer.parseInt(num2TextField.getText());
    }

    public void setResult(int result) {
        resultTextField.setText(String.valueOf(result));
    }

    public void addMathListener(MathListener listener) {
        addButton.addActionListener(listener);
        subtractButton.addActionListener(listener);
        multiplyButton.addActionListener(listener);
        divideButton.addActionListener(listener);
    }
}

MathListener.java

import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

public class MathListener implements ActionListener {
    private MathModel model;
    private MathView view;

    public MathListener(MathModel model, MathView view) {
        this.model = model;
        this.view = view;
        this.view.addMathListener(this);
    }

    @Override
    public void actionPerformed(ActionEvent e) {
        int num1 = view.getNum1();
        int num2 = view.getNum2();

        if (e.getSource() == view.addButton) {
            model.setNum1(num1);
            model.setNum2(num2);
            model.add();
        } else if (e.getSource() == view.subtractButton) {
            model.setNum1(num1);
            model.setNum2(num2);
            model.subtract();
        } else if (e.getSource() == view.multiplyButton) {
            model.setNum1(num1);
            model.setNum2(num2);
            model.multiply();
        } else if (e.getSource() == view.divideButton) {
            model.setNum1(num1);
            model.setNum2(num2);
            model.divide();
        }

        view.setResult(model.getResult());
    }
}

MathController.java

public class MathController {
    public static void main(String[] args) {
        MathModel model = new MathModel();
        MathView view = new MathView();
        new MathListener(model, view);
    }
}

在修改后的程序中，MathModel类负责处理数据的逻辑，MathView类负责显示用户界面，MathListener类实现对ActionEvent事件的监听，MathController类负责启动整个程序。通过MVC开发模式，将程序的不同职责分离开来，使得程序的设计更加清晰，易于维护和扩展。