}

    @Override
    public K getKey() {
      return key;
    }

    @Override
    public V getValue() {
      return value;
    }
  }

  public void testBuilderPutMutableEntryWithNullKeyFailsAtomically() {
    Builder<String, Integer> builder = new Builder<>();
    assertThrows(
        NullPointerException.class, () -> builder.put(new SimpleEntry<String, Integer>(null, 1)));