HashSet<String> set = new HashSet<String>();set.add("abc");

 1      private transient HashMap<E,Object> map;
 2     /**
 7      * Constructs a new, empty set; the backing <tt>HashMap</tt> instance has
 8      * default initial capacity (16) and load factor (0.75).
 9      */
10     public HashSet() {
11         map = new HashMap<>();
12     }

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

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

 1 final V putVal(int hash, K key, V value, boolean onlyIfAbsent,
 2                    boolean evict) {
 3         Node<K,V>[] tab; Node<K,V> p; int n, i;
 4         if ((tab = table) == null || (n = tab.length) == 0)
 5             n = (tab = resize()).length;
 6         if ((p = tab[i = (n - 1) & hash]) == null)
 7             tab[i] = newNode(hash, key, value, null);
 8         else {
 9             Node<K,V> e; K k;
10             if (p.hash == hash &&
11                 ((k = p.key) == key || (key != null && key.equals(k))))
12                 e = p;
13             else if (p instanceof TreeNode)
14                 e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
15             else {
16                 for (int binCount = 0; ; ++binCount) {
17                     if ((e = p.next) == null) {
18                         p.next = newNode(hash, key, value, null);
19                         if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st
20                             treeifyBin(tab, hash);
21                         break;
22                     }
23                     if (e.hash == hash &&
24                         ((k = e.key) == key || (key != null && key.equals(k))))
25                         break;
26                     p = e;
27                 }
28             }
29             if (e != null) { // existing mapping for key
30                 V oldValue = e.value;
31                 if (!onlyIfAbsent || oldValue == null)
32                     e.value = value;
33                 afterNodeAccess(e);
34                 return oldValue;
35             }
36         }
37         ++modCount;
38         if (++size > threshold)
39             resize();
40         afterNodeInsertion(evict);
41         return null;
42     }

1 static final int hash(Object key) {
2         int h;
3         return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);
4     }

 1  static class Node<K,V> implements Map.Entry<K,V> {
 2         final int hash;
 3         final K key;
 4         V value;
 5         Node<K,V> next;
 6
 7         Node(int hash, K key, V value, Node<K,V> next) {
 8             this.hash = hash;
 9             this.key = key;
10             this.value = value;
11             this.next = next;
12         }
13
14         public final K getKey()        { return key; }
15         public final V getValue()      { return value; }
16         public final String toString() { return key + "=" + value; }
17
18         public final int hashCode() {
19             return Objects.hashCode(key) ^ Objects.hashCode(value);
20         }
21
22         public final V setValue(V newValue) {
23             V oldValue = value;
24             value = newValue;
25             return oldValue;
26         }
27
28         public final boolean equals(Object o) {
29             if (o == this)
30                 return true;
31             if (o instanceof Map.Entry) {
32                 Map.Entry<?,?> e = (Map.Entry<?,?>)o;
33                 if (Objects.equals(key, e.getKey()) &&
34                     Objects.equals(value, e.getValue()))
35                     return true;
36             }
37             return false;
38         }
39     }