code: public static <K, V> Function<Map.Entry<K, V>, K> entryToKeyFunction() {
        return EntryToKeyFunction.getInstance();
     }
docs: Provides a function that returns the key of the entry when invoked.
  @param <K> key type of the entry
  @param <V> value type of the entry
  @return a function that when applied to a given entry will return the key