[Java知识库]重写hashCode和equals方法

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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：

Student{name='2号', age=12, id=202202}二本
Student{name='3号', age=13, id=202203}大专
Student{name='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));
    }
}

Student{name='2号', age=12, id=202202}二本
Student{name='3号', age=13, id=202203}大专
Student{name='1号', age=11, id=202201}一本
Student{name='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：

Student{name='2号', age=12, id=202202}二本
Student{name='1号', age=11, id=202201}一本
Student{name='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

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