一、HashMap

HashMap<Student3,String>
Map的唯一性指的是键的唯一性，HashMap中需要键的类型要重写hashCode()方法和equals方法

二、HashMap的使用

1.编写Student3类，里面需要重写hashCode()方法和equals方法

import java.util.Objects;

public class Student3 {
    private String name;
    private int age;

    public Student3() {
    }

    public Student3(String name, int age) {
        this.name = name;
        this.age = age;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public int getAge() {
        return age;
    }

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

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;
        Student3 student3 = (Student3) o;
        return age == student3.age && Objects.equals(name, student3.name);
    }

    @Override
    public int hashCode() {
        return Objects.hash(name, age);
    }

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

2.编写测试类

public class HashMapDemo1 {
    public static void main(String[] args) {
        HashMap<Student3, String> map1 = new HashMap<>();

        Student3 s1 = new Student3("李刚", 21);
        Student3 s2 = new Student3("钱志强", 22);
        Student3 s3 = new Student3("江川", 21);
        Student3 s4 = new Student3("李刚", 21);
        Student3 s5 = new Student3("吴问强", 23);

        map1.put(s1, "打羽毛球");
        map1.put(s2, "唱");
        map1.put(s3, "跳");
        map1.put(s4, "看美女");
        map1.put(s5, "打游戏");

        System.out.println("map1: " + map1);


    }
}

跟HashSet相似，都需要重写里面的方法