Map<String, String> map = transformValues(underlying, Functions.toStringFunction());
    assertEquals("1", map.remove("a"));
    assertThat(map.remove("b")).isNull();
  }

  public void testTransformEqualityOfMapsWithNullValues() {
    Map<String, @Nullable String> underlying = new HashMap<>();
    underlying.put("a", null);
    underlying.put("b", "");

    Map<String, Boolean> map =
        transformValues(