I have the following code snippet (Java 17, junit 5), and I am trying to get coverage on a test case where the inSellerList is null.

public class MyClass() {
  public myMethod {
    ResDTO resDto = doSomething();
  }

  private ResDTO doSomething() {
    ResDTO resDto = new ResDTO();
    // ...
    List<SellerDTO> editSellerListRes = editSellerList(resDto.getSellerList()); // <---- I need to mock this so that resDto.getSellerList() is null
    resDto.setSellerList(editSellerListRes);
    return resDto;
  }

  private List<SellerDTO> editSellerListRes editSellerList(List<SellerDTO> inSellerList) {
    if (inSellerList == null || inSellerList.size() == 0) {
      System.out.println("I need to have this part covered in the test")
      List<SellerDTO> = new ArrayList<>();
    }
  }
}

In my test scenario, I am using @MockBean to mock resDto.getSellerList so that it returns null, but based on the coverage report, the code inside the if block is not getting covered.

I am limited to only using junit 5 and mockito.

@SpringBootTest(classes = {MyClass.class})
class MyClassTest {
  @Autowired MyClass myClass;

  @MockBean
  private ResDTO resDto;

  @Test
  private void test1() {
  // does not work
  when(resDto.getSellerList()).thenReturn(null)
  // does not work either
  doAnswer(invocation -> {
    resDto.setSellerList(null);
    return null;
  }).when(resDto).setSellerList(anyList());

  myClass.doSomething()
  // assertions
  }
}