}

  public void testBuilderPutNullValue() {
    Builder<String, Integer> builder = new Builder<>();
    assertThrows(NullPointerException.class, () -> builder.put("one", null));
  }

  public void testBuilderPutNullKeyViaPutAll() {
    Builder<String, Integer> builder = new Builder<>();
    assertThrows(
        NullPointerException.class,
        () -> builder.putAll(Collections.<String, Integer>singletonMap(null, 1)));
  }