Method apply

guava-tests/test/com/google/common/collect/MapsTest.java:491–494

(String input)

            .onResultOf(
                new Function<String, String>() {
                  @Override
                  public String apply(String input) {
                    return input.toLowerCase();
                  }
                });

    MapDifference<Integer, String> diff1 = Maps.difference(left, right, caseInsensitiveEquivalence);