### Question:
CallbackMapper implements BiFunction<S, T, T> { @NonNull @Override public T apply(@NonNull final S source, @NonNull final T target) { T result = mapper.apply(source, target); callback.accept(source, result); return result; } CallbackMapper(@NonNull final BiFunction<S, T, T> mapper, @NonNull final BiConsumer<S, T> callback); @NonNull @Override T apply(@NonNull final S source, @NonNull final T target);  }### Answer:
@Test public void apply() { assertThat(mapper.apply(source, target)).isEqualTo(this.source + "/" + this.target); }