一文告诉你为什么要重写hashCode()方法和equals()方法

public native int hashCode();

public boolean equals(Object obj) {
        return (this == obj);
    }

JNIEXPORT jint JNICALL Java_java_lang_Object_hashCode
  (JNIEnv *, jobject);

package org;



public class Chengxuyuan {
    private Integer age;
    private String company;

    public Integer getAge() {
        return age;
    }

    public void setAge(Integer age) {
        this.age = age;
    }

    public String getCompany() {
        return company;
    }

    public void setCompany(String company) {
        this.company = company;
    }

    @Override
    public String toString() {
        return "Chengxuyuan{" +
                "age=" + age +
                ", company='" + company + '\'' +
                '}';
    }

//    @Override
//    public int hashCode() {
//        int hashcode = age.hashCode();
//        hashcode = 31 * hashcode + company.hashCode();
//        return hashcode;
//    }
//
//    @Override
//    public boolean equals(Object obj) {
//        if(!(obj instanceof Chengxuyuan)) {//首先要判断是不是同一个类型
//            return false;
//        }
//        Chengxuyuan chengxuyuan = (Chengxuyuan) obj;
//        if (this == chengxuyuan) {// 其次要判断地址是否相同相等
//            return true;
//        }
//        if (chengxuyuan.age.equals(this.age) && chengxuyuan.company.equals(this.company)) {// 最后要判断对象里的属性是否相同
//            return true;
//        } else {
//            return false;
//        }
//		}
}

package org;

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

public class Main {
    public static void main(String[] args) {
        Map<Chengxuyuan, String> map = new HashMap<>();
        Chengxuyuan chengxuyuan = new Chengxuyuan();
        chengxuyuan.setAge(15);
        chengxuyuan.setCompany("没公司");
        System.out.println(chengxuyuan.hashCode());
        map.put(chengxuyuan, chengxuyuan.getCompany());
        Chengxuyuan chengxuyuan2 = new Chengxuyuan();
        chengxuyuan2.setAge(15);
        chengxuyuan2.setCompany("没公司");
        System.out.println(chengxuyuan2.hashCode());
        map.put(chengxuyuan2, chengxuyuan2.getCompany());
        System.out.println(chengxuyuan.equals(chengxuyuan2));
        System.out.println(map);
        System.out.println(map.get(chengxuyuan2));
    }
}

public V put(K key, V value) {
        return putVal(hash(key), key, value, false, true);
    }

 
    public static int hashCode(Object a[]) {
        if (a == null)
            return 0;

        int result = 1;

        for (Object element : a)
            result = 31 * result + (element == null ? 0 : element.hashCode());

        return result;
    }

27392574
27392574
false
{Chengxuyuan{age=15, company='没公司'}=没公司, Chengxuyuan{age=15, company='没公司'}=没公司}
没公司