重写hashCode和equals方法

Posted 2022-04-19 top啦它

本文已参与「新人创作礼」活动，一起开启掘金创作之路。

首先写入三个学生，学生的唯一标识是学号。

import java.util.HashMap;

public class AADemo_2 
    public static void main(String[] args) 
        HashMap<Student,String> map = new HashMap<>();
        Student student1 = new Student("1号",11,202201);
        Student student2 = new Student("2号",12,202202);
        Student student3 = new Student("3号",13,202203);
        map.put(student1,"一本");
        map.put(student2,"二本");
        map.put(student3,"大专");
        map.forEach((key,value) -> System.out.println(key+value));
    


class Student
    String name;
    Integer age;
    Integer id;

    public Student(String name, Integer age, Integer id) 
        this.name = name;
        this.age = age;
        this.id = id;
    

    @Override
    public String toString() 
        return "Student" +
                "name='" + name + ''' +
                ", age=" + age +
                ", id=" + id +
                '';
    

输出如下：

Studentname='2号', age=12, id=202202二本
Studentname='3号', age=13, id=202203大专
Studentname='1号', age=11, id=202201一本

现在因为技术原因导致同一个学生被写入了两次

public class AADemo_2 
    public static void main(String[] args) 
        HashMap<Student,String> map = new HashMap<>();
        Student student1 = new Student("1号",11,202201);
        Student student2 = new Student("2号",12,202202);
        Student student3 = new Student("3号",13,202203);
        Student student4 = new Student("3号",13,202203);
        map.put(student1,"一本");
        map.put(student2,"二本");
        map.put(student3,"大专");
        map.put(student4,"大专");
        map.forEach((key,value) -> System.out.println(key+value));
    

Studentname='2号', age=12, id=202202二本
Studentname='3号', age=13, id=202203大专
Studentname='1号', age=11, id=202201一本
Studentname='3号', age=13, id=202203大专

领导说这种情况他不能容忍，要求程序员修改代码，否则就要扣工资
程序员想来想去，有了一个点子，重写hashcode和equals方法

package aa;

import java.util.HashMap;

public class AADemo_2 
    public static void main(String[] args) 
        Object a = new Object();
        HashMap<Student,String> map = new HashMap<>();
        Student student1 = new Student("1号",11,202201);
        Student student2 = new Student("2号",12,202202);
        Student student3 = new Student("3号",13,202203);
        Student student4 = new Student("3号",13,202203);
        map.put(student1,"一本");
        map.put(student2,"二本");
        map.put(student3,"大专");
        map.put(student4,"大专");
        map.forEach((key,value) -> System.out.println(key+value));
    


class Student
    String name;
    Integer age;
    Integer id;

    public Student(String name, Integer age, Integer id) 
        this.name = name;
        this.age = age;
        this.id = id;
    

    @Override
    public int hashCode() 
        if (this == null || !(this instanceof Student))
            return 0;
        
        int code = this.name.hashCode()+age+id;
        return code;
    

    @Override
    public boolean equals(Object obj) 
        if (obj == null || this == null || !(obj instanceof Student))
            return false;
        
        boolean flag = false;
        flag = this.hashCode() == obj.hashCode()?true:false;
        return flag;
    

    @Override
    public String toString() 
        return "Student" +
                "name='" + name + ''' +
                ", age=" + age +
                ", id=" + id +
                '';
    

运行结果：

Studentname='2号', age=12, id=202202二本
Studentname='1号', age=11, id=202201一本
Studentname='3号', age=13, id=202203大专

程序员把代码发给领导看了之后，领导非常满意，并决定不扣程序员的工资