code: public static <K,V> Map<K,V> singletonMap(K key, V value) {
          return new SingletonMap<>(key, value);
      }
docs: Returns an immutable map, mapping only the specified key to the
  specified value.  The returned map is serializable.
  
  @param <K> the class of the map keys
  @param <V> the class of the map values
  @param key the sole key to be stored in the returned map.
  @param value the value to which the returned map maps <tt>key</tt>.
  @return an immutable map containing only the specified key-value
  mapping.
  @since 1.3