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

下面是重构后的程序，按照MVC模式进行了设计：

Triangle.java （Model）

public class Triangle {
    private double a, b, c;

    public Triangle() {
        a = b = c = 0;
    }

    public void setA(double a) {
        this.a = a;
    }

    public void setB(double b) {
        this.b = b;
    }

    public void setC(double c) {
        this.c = c;
    }

    public double getArea() {
        double p = (a + b + c) / 2;
        return Math.sqrt(p * (p - a) * (p - b) * (p - c));
    }
}

WindowTriangle.java （View）

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

public class WindowTriangle extends JFrame {
    private JTextField txtA, txtB, txtC, txtArea;
    private JButton btnCalc;

    public WindowTriangle() {
        setTitle("Triangle");
        setSize(250, 150);
        setLocationRelativeTo(null);
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        JPanel p1 = new JPanel(new GridLayout(4, 2));
        p1.add(new JLabel("Side A:"));
        txtA = new JTextField(5);
        p1.add(txtA);
        p1.add(new JLabel("Side B:"));
        txtB = new JTextField(5);
        p1.add(txtB);
        p1.add(new JLabel("Side C:"));
        txtC = new JTextField(5);
        p1.add(txtC);
        p1.add(new JLabel("Area:"));
        txtArea = new JTextField(10);
        txtArea.setEditable(false);
        p1.add(txtArea);

        JPanel p2 = new JPanel();
        btnCalc = new JButton("Calculate");
        p2.add(btnCalc);

        getContentPane().setLayout(new BorderLayout());
        getContentPane().add(p1, BorderLayout.CENTER);
        getContentPane().add(p2, BorderLayout.SOUTH);
    }

    public double getA() {
        return Double.parseDouble(txtA.getText());
    }

    public double getB() {
        return Double.parseDouble(txtB.getText());
    }

    public double getC() {
        return Double.parseDouble(txtC.getText());
    }

    public void setArea(double area) {
        txtArea.setText(String.format("%.2f", area));
    }

    public void addMathListener(MathListener listener) {
        btnCalc.addActionListener(listener);
    }
}

MathListener.java （Controller）

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

public class MathListener implements ActionListener {
    private Triangle model;
    private WindowTriangle view;

    public MathListener(Triangle model, WindowTriangle view) {
        this.model = model;
        this.view = view;
    }

    @Override
    public void actionPerformed(ActionEvent e) {
        try {
            double a = view.getA();
            double b = view.getB();
            double c = view.getC();

            model.setA(a);
            model.setB(b);
            model.setC(c);

            double area = model.getArea();
            view.setArea(area);
        } catch (NumberFormatException ex) {
            JOptionPane.showMessageDialog(view, "Invalid input!", "Error", JOptionPane.ERROR_MESSAGE);
        }
    }
}

Example11_13.java （Main）

public class Example11_13 {
    public static void main(String[] args) {
        Triangle model = new Triangle();
        WindowTriangle view = new WindowTriangle();
        MathListener controller = new MathListener(model, view);

        view.addMathListener(controller);

        view.setVisible(true);
    }
}

重构后，程序按照MVC模式进行了设计，MathListener.java实现了对ActionEvent事件的监听，不再使用窗体做监视器对象。