重寫equals()時需要同時重寫hashcode()

Java強調當重寫equals()時需要同時重寫hashcode()方法,假設兩個不同對象，在內存中的地址不同分別爲a和b,那麼重寫equals()以後a.equals(b) =true 開發者希望把a,b這兩個key視作完全相等
然而由於內存地址的不同導致hashcode不同,會導致在hashmap中儲存2個本應相同的key值；
如果不被重寫（原生）的hashCode和equals是什麼樣的？
1. 不被重寫（原生）的hashCode值是根據內存地址換算出來的一個值。
2. 不被重寫（原生）的equals方法是嚴格判斷一個對象是否相等的方法（object1 == object2）。

示例代碼demo1：

1. 沒有重寫equals

package com.compareObject;

/**
 * @Auther: 13213
 * @Date: 2020/5/26 14:22
 * @Description:
 */
public class Dog {
    private int age;
    private String name;
    public Dog(int age,String name){
        this.age=age;
        this.name=name;
    }
    public int getAge() {
        return age;
    }
    public void setAge(int age) {
        this.age = age;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
//    @Override
//    public boolean equals(Object obj) {
//        if(obj==null){
//            return false;
//        }
//        if(this==obj){
//            return true;
//        }
//        if(obj instanceof Dog){
//            Dog dog=(Dog)obj;
//            if(dog.age==this.age&&dog.name.equals(this.name)){
//                return true;
//            }else{
//                return false;
//            }
//        }
//        return false;
//    }
    public static void main(String[] args){
        Dog dog1=new Dog(12,"wangcai");
        Dog dog2=new Dog(12,"wangcai");
        Dog dog3=new Dog(16,"wangcai");
        System.out.println(dog1.equals(dog2));//true
        System.out.println(dog1.equals(dog3));//false
    }
}

運行結果：

2. 重寫equals

package com.compareObject;

/**
 * @Auther: 13213
 * @Date: 2020/5/26 14:22
 * @Description:
 */
public class Dog {
    private int age;
    private String name;
    public Dog(int age,String name){
        this.age=age;
        this.name=name;
    }
    public int getAge() {
        return age;
    }
    public void setAge(int age) {
        this.age = age;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    @Override
    public boolean equals(Object obj) {
        if(obj==null){
            return false;
        }
        if(this==obj){
            return true;
        }
        if(obj instanceof Dog){
            Dog dog=(Dog)obj;
            if(dog.age==this.age&&dog.name.equals(this.name)){
                return true;
            }else{
                return false;
            }
        }
        return false;
    }
    public static void main(String[] args){
        Dog dog1=new Dog(12,"wangcai");
        Dog dog2=new Dog(12,"wangcai");
        Dog dog3=new Dog(16,"wangcai");
        System.out.println(dog1.equals(dog2));//true
        System.out.println(dog1.equals(dog3));//false
    }
}

運行結果：

示例代碼demo2:

package com.compareObject;

import java.util.Collection;
import java.util.HashSet;

/**
 * @Auther: 13213
 * @Date: 2020/5/26 14:22
 * @Description:
 */
public class Test {
    public static void main(String[] args) {
        Name n1 = new Name("zcj001");
        Name n2 = new Name("zcj001");
        Collection c = new HashSet();
        c.add(n1);
        c.add(n2);
        System.out.println(n1.equals(n2));
        System.out.println(n1.hashCode());
        System.out.println(n2.hashCode());
        System.out.println(c);
    }
}

class Name {
    private String id;
    public Name(String id) {
        this.id = id;
    }
    public String toString(){
        return this.id;
    }
    public boolean equals(Object obj) {
        if (obj instanceof Name) {
            Name name = (Name) obj;
            return (id.equals(name.id));
        }
        return super.equals(obj);
    }
    public int hashCode() {
        return id.hashCode();
    }
}

運行結果：

當只重寫equals()不重寫hashCode()運行結果如下：

示例代碼demo3：

package com.compareObject;
/**
 * @Auther: 13213
 * @Date: 2020/5/26 14:22
 * @Description:
 */
public class Test {
    public static void main(String[] args) {
        String a = "123";
        String b = "123";
        System.out.println(a.equals(b));
        System.out.println(a.hashCode());
        System.out.println(b.hashCode());
    }
}

運行結果：

此例說明，String對象重寫了equals()方法以及hashcode()方法，查看源代碼可知確實如此；