通过 Demo 理解 hashCode 与 equals 的关系

package com.heatdeath.object;

import lombok.extern.slf4j.Slf4j;

import java.util.HashMap;
import java.util.Map;

/**
 * Author:  heatdeath
 * Date:    2018/4/19
 * Desc:
 */
@Slf4j
public class EqualsDemo {

    public static void main(String[] args) {
        MyEqualsDemo demo_1 = new MyEqualsDemo("111", "111");
        MyEqualsDemo demo_2 = new MyEqualsDemo("222", "222");
        MyEqualsDemo demo_3 = new MyEqualsDemo("111", "111");

        log.info("demo_1 hashcode is {}", demo_1.hashCode());
        log.info("demo_2 hashcode is {}", demo_2.hashCode());
        log.info("demo_3 hashcode is {}", demo_3.hashCode());

        log.info("demo_1 toString is {}", demo_1.toString());
        log.info("demo_2 toString is {}", demo_2.toString());
        log.info("demo_3 toString is {}", demo_3.toString());

        log.info("demo_1 == demo_3 result is {}", demo_1 == demo_3);    // False
        log.info("demo_1.equals(demo_3) result is {}", demo_1.equals(demo_3));  // True

        log.info("-----------------------------------------------------");

//        Map<MyEqualsDemo, String> map = new HashMap<>();
//        map.put(demo_1, "demo_1");
//        log.info("map.get(demo_1) is {}", map.get(demo_1));
//
//        demo_1.setField_1("change field_1");
//        log.info("now demo_1 hashcode is {}", demo_1.hashCode());
//        log.info("map.get(demo_1) is {}", map.get(demo_1));
//
//        Map<int, String> map1 = new HashMap<>();
//        Map<String, int> map2 = new HashMap<>();

    }
}

class MyEqualsDemo {
    private String field_1;

    private String field_2;

    public MyEqualsDemo(String field_1, String field_2) {
        this.field_1 = field_1;
        this.field_2 = field_2;
    }

    public String getField_1() {
        return field_1;
    }

    public void setField_1(String field_1) {
        this.field_1 = field_1;
    }

    public String getField_2() {
        return field_2;
    }

    public void setField_2(String field_2) {
        this.field_2 = field_2;
    }

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;

        MyEqualsDemo that = (MyEqualsDemo) o;

        return (field_1 != null ? field_1.equals(that.field_1) : that.field_1 == null) && (field_2 != null ? field_2.equals(that.field_2) : that.field_2 == null);
    }

    @Override
    public int hashCode() {
        int result = field_1 != null ? field_1.hashCode() : 0;
        result = 31 * result + (field_2 != null ? field_2.hashCode() : 0);
        return result;
    }
}

21:33:30.309 [main] INFO com.heatdeath.object.EqualsDemo - demo_1 hashcode is 1557024
21:33:30.322 [main] INFO com.heatdeath.object.EqualsDemo - demo_2 hashcode is 1588800
21:33:30.322 [main] INFO com.heatdeath.object.EqualsDemo - demo_3 hashcode is 1557024
21:33:30.322 [main] INFO com.heatdeath.object.EqualsDemo - demo_1 toString is com.heatdeath.object.MyEqualsDemo@17c220
21:33:30.322 [main] INFO com.heatdeath.object.EqualsDemo - demo_2 toString is com.heatdeath.object.MyEqualsDemo@183e40
21:33:30.322 [main] INFO com.heatdeath.object.EqualsDemo - demo_3 toString is com.heatdeath.object.MyEqualsDemo@17c220
21:33:30.322 [main] INFO com.heatdeath.object.EqualsDemo - demo_1 == demo_3 result is false
21:33:30.322 [main] INFO com.heatdeath.object.EqualsDemo - demo_1.equals(demo_3) result is true
21:33:30.322 [main] INFO com.heatdeath.object.EqualsDemo - -----------------------------------------------------

结论：

1、如果 demo_1.equals(demo_3) 为 true，则 demo_1 与 demo_3 的 hashCode 必定相同

2、如果两个对象的 hashCode 相同，但 equals() 不一定返回 true

3、 equals 比较的是对象的内容， == 比较的是引用类型数据所指向堆中对象的地址

这个是别人的结论，好像比我的更精炼一些…

1）对于 == 关系操作符，如果作用于基本数据类型的变量，则直接比较其存储的 “值”是否相等；

如果作用于引用类型的变量，则比较的是所指向的对象的地址

2）对于 equals 方法，注意： equals 方法不能作用于基本数据类型的变量

如果没有对 equals 方法进行重写，则比较的是引用类型的变量所指向的对象的地址；

诸如 String、Date 等类对 equals 方法进行了重写的话，比较的是所指向的对象的内容。