code: public static <K, V> Function<Map.Entry<K, V>, K> entryToKeyFunction() {
        return (Function) entryToKeyFunction;
     }
docs: Provides a {@link Function} that can be used to convert from an instance of {@link java.util.Map.Entry} to
  the key of the entry. This is useful for the instance passed to a method like {@link Flowable#map(Function)}.
  @param <K> key type
  @param <V> value type
  @return rxjava function to convert from a Map.Entry to its key.