code: static <K, V> Entry<K, V> entryOf(K key, V value) {
      checkEntryNotNull(key, value);
      return new AbstractMap.SimpleImmutableEntry<>(key, value);
    }
docs: Verifies that {@code key} and {@code value} are non-null, and returns a new immutable entry
  with those values.
  
  <p>A call to {@link Entry#setValue} on the returned entry will always throw {@link
  UnsupportedOperationException}.