code: public V putIfAbsent( K key, V value )
    {
      if( value == null )
      {
        throw new NullPointerException();
      }
      int hash = hash( key.hashCode() );
      return segmentFor( hash ).put( key, hash, value, true );
    }
docs: {@inheritDoc}
  
  @return the previous value associated with the specified key,
  or <tt>null</tt> if there was no mapping for the key
  
  @throws NullPointerException if the specified key or value is null