Java中如何正確重寫equals方法

//String類equals源代碼：
public boolean equals(Object anObject) {
        if (this == anObject) {
            return true;
        }
        if (anObject instanceof String) {
            String anotherString = (String)anObject;
            int n = value.length;
            if (n == anotherString.value.length) {
                char v1[] = value;
                char v2[] = anotherString.value;
                int i = 0;
                while (n-- != 0) {
                    if (v1[i] != v2[i])
                        return false;
                    i++;
                }
                return true;
            }
        }
        return false;
    }

//Integer類的equals源代碼:
 public boolean equals(Object obj) {
        if (obj instanceof Integer) {
            return value == ((Integer)obj).intValue();
        }
        return false;
    }

package com.test10_04;

import java.util.Objects;

class Rectangle {
    private double length;
    private double wide;

    public Rectangle() {
        //空實現
    }
    public Rectangle(double length, double wide) {
        setLength(length);
        setWide(wide);
    }

    public double getLength() {
        return length;
    }

    public void setLength(double length) {
        assert length > 0.0 : "您的輸入有誤，長方形的長不能小於0";
        this.length = length;
    }

    public double getWide() {
        return wide;
    }

    public void setWide(double wide) {
        assert wide > 0.0 : "您的輸入有誤，長方形的寬不能小於0";
        this.wide = wide;
    }

    public double area() {
        return this.length * this.wide;

    }
    public double circumference() {
        return 2 * (this.wide + this.length);
    }

    public boolean equals(Object obj) {
        if (this == obj) { //判斷一下如果是同一個對象直接返回true，提高效率
            return true;
        }
        if (obj == null || obj.getClass() != this.getClass()) { //如果傳進來的對象為null或者二者為不同類，直接返回false
            return false;
        }
        //也可以以下方法：
//        if (obj == null || !(obj instanceof Rectangle)) { //如果傳進來的對象為null或者二者為不同類，直接返回false
//            return false;
//        }
        Rectangle rectangle = (Rectangle) obj; //向下轉型
        //比較長寬是否相等，註意：浮點數的比較不能簡單地用==，會有精度的誤差，用Math.abs或者Double.compare
        return Double.compare(rectangle.length, length) == 0 && Double.compare(rectangle.wide, wide) == 0;
    }

    public int hashCode() { //重寫equals的同時也要重寫hashCode，因為同一對象的hashCode永遠相等
        return Objects.hash(length, wide); //調用Objects類，這是Object類的子類
    }

    public String toString() {
        return "Rectangle{" + "length=" + length + ", wide=" + wide + '}';
    }
}

public class TestDemo {
    public static void main(String[] args) {

        Rectangle rectangle1 = new Rectangle(3.0, 2.0);
        Rectangle rectangle2 = new Rectangle(3.0, 2.0);
        
        System.out.println(rectangle1.equals(rectangle2));
        System.out.println("rectangle1哈希碼:" + rectangle1.hashCode() +
                "\nrectangle2哈希碼:" + rectangle2.hashCode());
        System.out.println("toString打印信息：" + rectangle1.toString());
    }
}

//getClass()版本
public class Student {
	private String name;
	
    public void setName(String name) {
        this.name = name;
    }
	@Override
	public boolean equals(Object object){
		if (object == this)
			return true;
		// 使用getClass()判斷對象是否屬於該類
		if (object == null || object.getClass() != getClass())
			return false;
		Student student = (Student)object;
		return name != null && name.equals(student.name);
}

//instanceof版本
public class Student {
	private String name;
	
    public void setName(String name) {
       this.name = name;
   }
	@Override
	public boolean equals(Object object){
		if (object == this)
			return true;
		// 通過instanceof來判斷對象是否屬於類
		if (object == null || !(object instanceof Student))
			return false;
		Student student = (Student)object;
		return name!=null && name.equals(student.name);
	}
}

class GoodStudent extends Student {

    @Override
    public boolean equals(Object object) {
        return false;
    }

    public static void main(String[] args) {
        GoodStudent son = new GoodStudent();
        Student father = new Student();

        son.setName("test");
        father.setName("test");

		// 當使用instance of時
        System.out.println(son.equals(father)); // 這裡為false
        System.out.println(father.equals(son)); // 這裡為true

		// 當使用getClass()時
        System.out.println(son.equals(father)); // 這裡為false
        System.out.println(father.equals(son)); // 這裡為false	
    }
}