code: public V put(K key, V value) {
      EntryImpl<K, V> entry = _entries[keyHash(key) & _mask];
      while (entry != null) {
        if (key.equals(entry._key)) {
          V prevValue = entry._value;
          entry._value = value;
          return prevValue;
        }
        entry = entry._next;
      }
      // No previous mapping.
      addEntry(key, value);
      return null;
    }
docs: Associates the specified value with the specified key in this {@link FastMap}. If the
  {@link FastMap} previously contained a mapping
  for this key, the old value is replaced.
  
  @param key the key with which the specified value is to be associated.
  @param value the value to be associated with the specified key.
  @return the previous value associated with specified key,
  or <code>null</code> if there was no mapping for key.
  A <code>null</code> return can also indicate that the map
  previously associated <code>null</code> with the specified key.
  @throws NullPointerException if the key is <code>null</code>.