How null key is handled in HashMap ?

equals() and hashCode() are used to store and retrieve values, but null is handled in different way using 2 different methods : 

  1. putForNullKey(V value)

  2. getForNullKey()

Null keys always map to index 0.

Internal implementation

private V getForNullKey() {

   for (Entry<K,V> e = table[0]; e != null; e = e.next) {

      if (e.key == null)

         return e.value;

   }

   return null;

}

/**

 * Offloaded version of put for null keys

 */

private V putForNullKey(V value) {

   for (Entry<K,V> e = table[0]; e != null; e = e.next) {

      if (e.key == null) {

         V oldValue = e.value;

         e.value = value;

         e.recordAccess(this);

         return oldValue;

      }

   }

   modCount++;

   addEntry(0, null, value, 0);

   return null;

}