code: public V put(K key, V value) {
          return putVal(hash(key), key, value, false, true);
      }
docs: Associates the specified value with the specified key in this map.
  If the map previously contained a mapping for the key, the old
  value is replaced.
  
  @param key key with which the specified value is to be associated
  @param value value to be associated with the specified key
  @return the previous value associated with <tt>key</tt>, or
  <tt>null</tt> if there was no mapping for <tt>key</tt>.
  (A <tt>null</tt> return can also indicate that the map
  previously associated <tt>null</tt> with <tt>key</tt>.)