Details the add method of the HashSet class (four)

Override the hashCode method in a custom class

public class Student {
    private String id;
    
    public Student(String id) {    
        this.id=id;
    }

    @Override
    public int hashCode() {         return id.hashCode();//Return the hash value of id     } }

 

import java.util.HashSet;

public class Test1 {

    public static void main(String[] args) {
        HashSet<Student> set = new HashSet<Student>();
        System.out.println(set.add(new Student("100")));//true
        System.out.println(set.add(new Student("100")));//true
    }

}

 Add method in HashSet

public boolean add(E e) {
        return map.put(e, PRESENT)==null;
    }

 Put method in HashMap

public V put(K key, V value) {
        return putVal(hash(key), key, value, false, true);
    }

PutVal method in HashMap

final V putVal(int hash, K key, V value, boolean onlyIfAbsent,
                   boolean evict) {         Node<K,V>[] tab; Node<K,V> p; int n, i;         if ((tab = table) == null || (n = tab.length) == 0)             n = (tab = resize()).length;         if ((p = tab[i = (n-1) & hash]) == null) //Execute the hashCode method in Student when adding it for the second time, the return value is the same             tab[i] = newNode(hash, key, value, null); //do not execute         else {             Node<K,V> e; K k;             if (p.hash == hash &&                 ((k = p.key) == key || (key != null && key.equals(k))))//equals method is not overridden, and the return value of equals method false                 e = p; //Do not execute










            else if (p instanceof TreeNode)
                e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
            else {
                for (int binCount = 0; ; ++binCount) {
                    if ((e = p.next) == null) {
                        p.next = newNode(hash, key, value, null);
                        if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st
                            treeifyBin(tab, hash);
                        break;
                    }
                    if (e.hash == hash &&
                        ((k = e.key) == key || (key != null && key.equals(k))))
                        break;
                    p = e;
                }
            }
            if (e != null) { // existing mapping for key
                V oldValue = e.value;
                if (!onlyIfAbsent || oldValue == null)
                    e.value = value;
                afterNodeAccess(e);
                return oldValue;
            }
        }
        ++modCount;
        if (++size > threshold)
            resize();
        afterNodeInsertion(evict);
        return null;//返回null
    }

putVal method returns null

put method returns null

The add method returns true, the addition is successful