帮你搞懂Java中重写equals方法为什么要重写hashcode方法？

话不多说，直接上例子，包你懂

首先我们只重新equals()方法

public class Student {

    private String name;
    private  int age;
    private  String QQ;

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;
        Student student = (Student) o;
        return age == student.age &&
                Objects.equals(name, student.name) &&
                Objects.equals(QQ, student.QQ);
    }

看我们的测试类

public class Test {
    public static void main(String[] args) {
        Student student = new Student();
        Student student2 = new Student();
        System.out.println(student.equals(student2));
        System.out.println(student.hashCode());
        System.out.println(student2.hashCode());
        HashMap<Student, String> map = new HashMap<>();
        map.put(student,"123");
        map.put(student2,"456");
        System.out.println(map.get(student));
        System.out.println(map.get(student2));

    }
}

依次输出

true                               
356573597             student 的hashcode值
1735600054            student 2的hashcode值
123                         
456

是否出现矛盾？？？
用equals比较说明对象相同，但是在HashMap中却以不同的对象存储（没有重写hascode值，两个hascode值，在他看来就是两个对象）。
到底这两个对象相等不相等？？？？
说明必须重写hashCode()的重要性，

接下来重写重写equals方法和hashCode方法,再比较

public class Student {

    private String name;
    private  int age;
    private  String QQ;

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;
        Student student = (Student) o;
        return age == student.age &&
                Objects.equals(name, student.name) &&
                Objects.equals(QQ, student.QQ);
    }

    @Override
    public int hashCode() {

        return Objects.hash(name, age, QQ);
    }
}

测试类

public class Test {
    public static void main(String[] args) {
        Student student = new Student();
        Student student2 = new Student();
        System.out.println(student.equals(student2));
        System.out.println(student.hashCode());
        System.out.println(student2.hashCode());
        HashMap<Student, String> map = new HashMap<>();
        map.put(student,"123");
        map.put(student2,"456");
        System.out.println(map.get(student));
        System.out.println(map.get(student2));

    }
}

依次输出

true
29791           //相同的对象
29791
456			  //说明以一个值key存储，相同的值
456

看到这里，同学 你懂了吗？ 还不懂，可以自己实现一遍代码。