Talking about: is the HashSet set ordered or disordered /hashset set unique, why can we store elements with the same content

package Set;
import java.util.Collections;
import java.util.HashSet;
public class HashSetDemo01 {
    
    public static void main(String[] args) {
    
        //HashSet The underlying data structure of a collection is a hash table , It is a secondary data structure , The hash table is composed of an array + Linked list implementation of 
        //Set The characteristic of a set is that the elements are unique , Because in its system, although HashSet The order in which the set accesses data is out of order ( Of course, it may also be orderly ),
        //  But through in-depth analysis , We can actually see , The order of our cognition refers to the order of access ,HashSet The set is based on hash value pairs 16 Take the rest to decide which bucket to store , There is also a sequence of going deep into the bottom 
        //HashSet The collection has 0~15 An array of indexes of .
        HashSet<dog> hashSet = new HashSet<>();
        Collections.addAll(hashSet,new dog(" Darling ",1),new dog(" Recruit money ",3),new dog(" Recruit money ",3));
        // We store three elements in the collection , Suppose that the two incoming fortunes are the same dog , But by printing the set, we can see that both dogs have been added 
        System.out.println(hashSet);
        ![ Print set results ](https://img-blog.csdnimg.cn/209db4aa0ea1419b9f4f43fae96ecbd2.png#pic_center)

        // reflection :HashSet The set should be unique to the element , However, elements with the same content can be added to the collection , Why does this happen here ？
        // We need to know a method first  public int hashCode(), This method is Object Class , Any object calls hashCode Methods will return the corresponding hash value , The default is generated according to the address value 
        // What we need to know , because int The data is -2147483648~2147483647, If there are enough elements , Object call hashCode The data obtained by the method will certainly be repeated , This is called hash collision .
        //HashSet The underlying data structure of a collection is an array + Linked list , When creating this collection , It initializes a with a length of 16 Array of , Each array stores linked list nodes . Its storage characteristics are determined by the hash value , It will make every element call hashCode Method , Make hash value pairs 16 Remainder , The remainder is stored in the bucket , Judge whether there are elements in the bucket , If there is no element , It is stored as the head node of the current bucket 
        // If there are elements in the bucket ,(1) First judge and compare the hash value of this element with the hash value of the bucket , Even if the hash values of two elements are the same , The system will not think that elements are repeated , Because hash collision may occur , Although the possibility is relatively small , But it can also happen 
        //(2) Next we will use equals Method to compare the address value of the element to be stored with the address value of the element in the bucket , Because the objects are new Coming out , The address in the heap space is unique , So whether the content is the same or not , Will be stored in the collection 

        // Here we can make it clear ,hashSet Collections store data elements , Is determined by the hash value , The underlying hash value is determined by the address value . Originally in hashSet Determine whether the elements are repeated in the set   In the final analysis, it is to judge the address value of two elements !
        // And we don't want to add elements through address values at all , Instead, I want to judge the content .

        // Solutions :
        // Override in the class of the object element equals as well as hashCode Method .
        // The rewritten equals The bottom layer of the method is to compare the content ,hashCode Methods are also generated based on content .
        // After rewriting , Set stored data , It still makes every element call hashCode Method to get the hash value , We need to know that the current hash value is generated through content , So if there are elements in the bucket to be stored , Judge whether the hash value of the element in the bucket is the same , If it's different, deposit it directly ,
        // If the same, it will be considered that hash collision occurs ,(2) Use equals Method to judge the content between elements , If different, it is stored in the set , If true It is clear that this is a repeating element !
![dog Class to equals and hashCode Method ](https://img-blog.csdnimg.cn/fe43f68905894a228cd910e287925379.png?x-oss-process=image/watermark,type_d3F5LXplbmhlaQ,shadow_50,text_Q1NETiBAYmFpMjU5MjU3,size_20,color_FFFFFF,t_70,g_se,x_16#pic_center)
}
}