在上一题Point2D和Point3D类的基础上，新建一个TestPointV2类，在TestPointV2类的main()方法中添加如下语句。 Scanner sc = new Scanner(System.in); System.out.println("Please enter the coordinates of p23:"); double p23x = sc.nextDouble(); double p23y = sc.nextDouble(); Point2D p23 = new Point2D(p23x, p23y); System.out.println("Please enter the coordinates of p31:"); double p31x = sc.nextDouble(); double p31y = sc.nextDouble(); double p31z = sc.nextDouble(); Point3D p33 = new Point3D(p31x, p31y, p31z); System.out.println("Please enter the coordinates of p24:"); double p24x = sc.nextDouble(); double p24y = sc.nextDouble(); double p24z = sc.nextDouble(); sc.close(); // The reference of the parent class refers to the object of the subclass. Point2D p24 = new Point3D(p24x, p24y, p24z); System.out.println("Does " + p23 + " coincide with " + p33 + "? -- "+ p23.equals(p33)); System.out.println("Does " + p33 + " coincide with " + p23 + "? -- "+ p33.equals(p23)); System.out.println("Does " + p33 + " coincide with " + p24 + "? -- "+ p33.equals(p24)); System.out.println("Does " + p24 + " coincide with " + p33 + "? -- "+ p24.equals(p33)); System.out.println("Does " + p23 + " coincide with " + p24 + "? -- "+ p23.equals(p24)); System.out.println("Does " + p24 + " coincide with " + p23 + "? -- "+ p24.equals(p23)); 假设引用变量p23、p33和p24所指点对象的坐标依次为(0, 0)，(0, 0, 5)，(0, 0, 5)。从键盘输入这三个点的坐标值，上述语句的运行结果如下： Please enter the coordinates of p23: 0 0 Please enter the coordinates of p31: 0 0 5 Please enter the coordinates of p24: 0 0 5 Does (0.0, 0.0) coincide with (0.0, 0.0, 5.0)? -- true Does (0.0, 0.0, 5.0) coincide with (0.0, 0.0)? -- true Does (0.0, 0.0, 5.0) coincide with (0.0, 0.0, 5.0)? -- true Does (0.0, 0.0, 5.0) coincide with (0.0, 0.0, 5.0)? -- true Does (0.0, 0.0) coincide with (0.0, 0.0, 5.0)? -- true Does (0.0, 0.0, 5.0) coincide with (0.0, 0.0)? -- true 该结果显然不符合事实，请分析原因并改进Point2D类的代码，使得上述TestPointV2类的代码能够得到正确的运行结果。

上述运行结果不正确的原因在于 Point2D p24 = new Point3D(p24x, p24y, p24z); 这一语句，将一个 Point3D 对象赋值给了 Point2D 类型的引用变量 p24，导致在调用 p24.equals(p33) 时出现了错误的结果。

为了解决这个问题，我们需要在 Point2D 类中添加一个 equals 方法，用于比较两个点是否相同。具体代码如下：

public boolean equals(Object obj) {
    if (obj instanceof Point2D) {
        Point2D other = (Point2D) obj;
        return this.x == other.x && this.y == other.y;
    }
    return false;
}

这个 equals 方法首先判断参数是否为 Point2D 类型，如果是，再将其转换为 Point2D 类型进行比较。这样就可以避免上述问题的出现。

修改后的 Point2D 类的完整代码如下：

public class Point2D {
    protected double x;
    protected double y;

    public Point2D() {
        this(0, 0);
    }

    public Point2D(double x, double y) {
        this.x = x;
        this.y = y;
    }

    public double getX() {
        return x;
    }

    public double getY() {
        return y;
    }

    public void setX(double x) {
        this.x = x;
    }

    public void setY(double y) {
        this.y = y;
    }

    public boolean equals(Object obj) {
        if (obj instanceof Point2D) {
            Point2D other = (Point2D) obj;
            return this.x == other.x && this.y == other.y;
        }
        return false;
    }
}

修改后的 TestPointV2 类的完整代码如下：

import java.util.Scanner;

public class TestPointV2 {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);

        System.out.println("Please enter the coordinates of p23:");
        double p23x = sc.nextDouble();
        double p23y = sc.nextDouble();
        Point2D p23 = new Point2D(p23x, p23y);

        System.out.println("Please enter the coordinates of p31:");
        double p31x = sc.nextDouble();
        double p31y = sc.nextDouble();
        double p31z = sc.nextDouble();
        Point3D p33 = new Point3D(p31x, p31y, p31z);

        System.out.println("Please enter the coordinates of p24:");
        double p24x = sc.nextDouble();
        double p24y = sc.nextDouble();
        double p24z = sc.nextDouble();
        sc.close();

        // The reference of the parent class refers to the object of the subclass.
        Point2D p24 = new Point3D(p24x, p24y, p24z);

        System.out.println("Does " + p23 + " coincide with " + p33 + "? -- " + p23.equals(p33));
        System.out.println("Does " + p33 + " coincide with " + p23 + "? -- " + p33.equals(p23));
        System.out.println("Does " + p33 + " coincide with " + p24 + "? -- " + p33.equals(p24));
        System.out.println("Does " + p24 + " coincide with " + p33 + "? -- " + p24.equals(p33));
        System.out.println("Does " + p23 + " coincide with " + p24 + "? -- " + p23.equals(p24));
        System.out.println("Does " + p24 + " coincide with " + p23 + "? -- " + p24.equals(p23));
    }
}