### Question:
ProblemTransformer { public List<ProblemDTO> toDtos(Collection<Problem> problems) { if (null == problems) { return new ArrayList<>(); } return problems.stream() .map(this::toDto) .collect(Collectors.toList()); }  List<ProblemDTO> toDtos(Collection<Problem> problems);  }### Answer:
@Test public void toDtos_should_return_empty_list_on_empty_list() { List<ProblemDTO> result = cut.toDtos(new ArrayList<>()); Assertions.assertThat(result).isNotNull(); Assertions.assertThat(result).isEmpty(); }
@Test public void toDtos_should_return_empty_list_on_null() { List<ProblemDTO> result = cut.toDtos(null); Assertions.assertThat(result).isNotNull(); Assertions.assertThat(result).isEmpty(); }