Map学习二之HashMap唯一性的解释

package com.itheima.demo02.Map;

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

/*

HashMap存储自定义类型键值
Map集合保证key是唯一的:
    作为key的元素,必须重写hashCode方法和equals方法,以保证key唯一

*/
public class Demo01HashMapSavePerson {

public static void main(String[] args) {
    show02();
}

/*
    HashMap存储自定义类型键值
    key:Person类型
        Person类就必须重写hashCode方法和equals方法,以保证key唯一
    value:String类型
        可以重复
 */
private static void show02() {
    //创建HashMap集合
    HashMap map = new HashMap<>();
    //往集合中添加元素
    map.put(new Person("女王",18),"英国");
    map.put(new Person("秦始皇",18),"秦国");
    map.put(new Person("普京",30),"俄罗斯");
    map.put(new Person("女王",18),"毛里求斯");
    //使用entrySet和增强for遍历Map集合
    Set> set = map.entrySet();
    for (Map.Entry entry : set) {
        Person key = entry.getKey();
        String value = entry.getValue();
        System.out.println(key+"-->"+value);
    }
}

/*
    HashMap存储自定义类型键值
    key:String类型
        String类重写hashCode方法和equals方法,可以保证key唯一
    value:Person类型
        value可以重复(同名同年龄的人视为同一个)
 */
private static void show01() {
    //创建HashMap集合
    HashMap map = new HashMap<>();
    //往集合中添加元素
    map.put("北京",new Person("张三",18));
    map.put("上海",new Person("李四",19));
    map.put("广州",new Person("王五",20));
    map.put("北京",new Person("赵六",18));
    //使用keySet加增强for遍历Map集合
    Set set = map.keySet();
    for (String key : set) {
        Person value = map.get(key);
        System.out.println(key+"-->"+value);
    }
}

}

package com.itheima.demo02.Map;

import java.util.Objects;

public class Person {

private String name;
private  int age;

public Person() {
}

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

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

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

@Override
public int hashCode() {

    return Objects.hash(name, 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;
}

}