HashSet (rewrite hashCode and equals)

I. Overview

There are two sub-interfaces under Collection:
List: ArrayList LinkedList Vector Repeatable, ordered
Set: HashSet The bottom layer is a hash table and the storage efficiency is quite high. The
bottom layer of TreeSet is a tree structure. The data must be stored in a comparative order. Natural order.

There is no unique method under the Set interface, you can use the method under Collection

2. Example (rewrite hashCode and equals):

import java.util.HashSet;
import java.util.Set;

/**
 * @author xue_yun_xiang
 * @create 2021-03-16-20:39
 */

class Cat {
    
    
    int age;
    String name;

    public Cat() {
    
    

    }

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

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

    @Override
    public int hashCode() {
    
    
        return age;
    }

    @Override
    public boolean equals(Object obj) {
    
    
        if (this == obj) {
    
    
            return true;
        }

        Cat c = (Cat)obj;
        return this.age == c.age && this.name.equals(c.name);
    }
}
public class Demo4 {
    
    
    public static void main(String[] args) {
    
    
        HashSet<Cat> list = new HashSet<>();

        Cat c1 = new Cat(1, "狸花猫");
        Cat c2 = new Cat(1, "狸花猫");

        list.add(c1);
        list.add(c2);

        //重写方法之前：
        //System.out.println(list);//[Cat{age=1, name='狸花猫'}, Cat{age=1, name='狸花猫'}]
        //System.out.println(c1.hashCode());//460141958
        //System.out.println(c2.hashCode());//1163157884
        //此时两个内容完全一样的两个对象都存进了HashSet,给人的表象是可以存入重复的数据，这样是不合理的

        //只重写hashCode()
        //System.out.println(list);//[Cat{age=1, name='狸花猫'}, Cat{age=1, name='狸花猫'}]
        //System.out.println(c1.hashCode());//1
        //System.out.println(c2.hashCode());//1
        //此时，虽然把表象的hash改为相等的，但仍执行Object类的equals方法，而此方法是严格的，
        // 会比较物理地址，而p1,p2是两个不同的物理地址，所以还会可以把p1,p2存入HashSet

        //只重写equals()
        //System.out.println(list);//[Cat{age=1, name='狸花猫'}, Cat{age=1, name='狸花猫'}]
        //System.out.println(c1.hashCode());//460141958
       // System.out.println(c2.hashCode());//1163157884
        //此时，虽然重写了equals，改成了只比较内容，但hash值不同，就不再执行equals方法

        //同时重写hashCode() 和 equals()
        System.out.println(list);//[Cat{age=1, name='狸花猫'}]
        System.out.println(c1.hashCode());//1
        System.out.println(c2.hashCode());//1
        //此时才符合HashSet数据不能重复的特性
    }
}