根据以下要求：Instead of using a binary file to save the arraylist of points, change the savaData method and the constructor of the Model class to use a database to write / read the coordinates of all the points. Use XAMPP and phpMyAdmin to create a database called "java" with a table called "points" that has two integer columns x and y (in addition to the ID primary key). Hint: make sure you delete all the old point coordinates from the database before inserting new ones. Hint: use phpMyAdmin to check what is stored in the database。修改下述代码：public class Model implements Serializable { private ArrayList<Point> points; private ArrayList<ModelListener> listeners; private static final String FILE_NAME = "points.bin"; public Model() { points = new ArrayList<Point>(); listeners = new ArrayList<ModelListener>(); // Read points from file if it exists File file = new File(FILE_NAME); if (file.exists()) { try { ObjectInputStream in = new ObjectInputStream(new FileInputStream(file)); points = (ArrayList<Point>) in.readObject(); in.close(); } catch (IOException e) { e.printStackTrace(); } catch (ClassNotFoundException e) { e.printStackTrace(); } } } public void addListener(ModelListener l) { listeners.add(l); } public ArrayList<Point> getPoints() { return points; } public void addPoint(Point p) { points.add(p); notifyListeners(); // points changed so notify the listeners. saveData(); // save point to file } public void clearAllPoints() { points.clear(); notifyListeners(); // points changed so notify the listeners. saveData(); // save empty list to file } public void deleteLastPoint() { if (points.size() > 0) { points.remove(points.size() - 1); notifyListeners(); // points changed so notify the listeners. saveData(); // save updated list to file } } private void notifyListeners() { for (ModelListener l : listeners) { l.update(); // Tell the listener that something changed. } } public int numberOfPoints() { return points.size(); } public void saveData() { try { ObjectOutputStream out = new ObjectOutputStream(new FileOutputStream(FILE_NAME)); out.writeObject(points); out.close(); } catch (IOException e) { e.printStackTrace(); } } }

修改后的代码如下所示，主要是将数据存储方式从二进制文件改为了使用数据库。具体实现是创建了一个名为"java"的数据库，并在其中创建了一个名为"points"的表，该表有三个列，分别是ID、x和y，其中ID是主键。在saveData()方法中，使用JDBC连接数据库，并将所有点的坐标存储到"points"表中。在Model类的构造函数中，读取数据库中保存的所有点的坐标，并将其存储在points列表中。

import java.sql.*;
import java.util.ArrayList;

public class Model implements Serializable {
    private ArrayList<Point> points;
    private ArrayList<ModelListener> listeners;
    private static final String DB_URL = "jdbc:mysql://localhost:3306/java";
    private static final String DB_USER = "root";
    private static final String DB_PASSWORD = "password";

    public Model() {
        points = new ArrayList<Point>();
        listeners = new ArrayList<ModelListener>();

        // Read points from database
        try {
            Connection conn = DriverManager.getConnection(DB_URL, DB_USER, DB_PASSWORD);
            Statement stmt = conn.createStatement();
            ResultSet rs = stmt.executeQuery("SELECT * FROM points");

            while (rs.next()) {
                int x = rs.getInt("x");
                int y = rs.getInt("y");
                points.add(new Point(x, y));
            }

            rs.close();
            stmt.close();
            conn.close();
        } catch (SQLException e) {
            e.printStackTrace();
        }
    }

    public void addListener(ModelListener l) {
        listeners.add(l);
    }

    public ArrayList<Point> getPoints() {
        return points;
    }

    public void addPoint(Point p) {
        points.add(p);
        notifyListeners(); // points changed so notify the listeners.
        saveData(); // save point to database
    }

    public void clearAllPoints() {
        points.clear();
        notifyListeners(); // points changed so notify the listeners.
        saveData(); // save empty list to database
    }

    public void deleteLastPoint() {
        if (points.size() > 0) {
            points.remove(points.size() - 1);
            notifyListeners(); // points changed so notify the listeners.
            saveData(); // save updated list to database
        }
    }

    private void notifyListeners() {
        for (ModelListener l : listeners) {
            l.update(); // Tell the listener that something changed.
        }
    }

    public int numberOfPoints() {
        return points.size();
    }

    public void saveData() {
        try {
            Connection conn = DriverManager.getConnection(DB_URL, DB_USER, DB_PASSWORD);
            Statement stmt = conn.createStatement();
            stmt.executeUpdate("DELETE FROM points");
            for (Point p : points) {
                stmt.executeUpdate("INSERT INTO points (x, y) VALUES (" + p.x + ", " + p.y + ")");
            }
            stmt.close();
            conn.close();
        } catch (SQLException e) {
            e.printStackTrace();
        }
    }
}