org.opentest4j.AssertionFailedError: Unexpected exception type thrown ==> expected

I am trying to test a method sendEmail. I would like to do a negative test on the method. I want to catch the IllegalStateException case. The sendemail method is in my ServiceClient class. In the test I am mocking serviceClient and apiClient the code for the method looks like this:

public HttpStatus sendEmail(){
        Supplier<HttpStatus> apiRequest = apiclient.post(command);
        return retry(apiRequest).orElseThrow(() -> new IllegalStateException());
}

private <T> Optional<T> retry(Supplier<T> apiRequest) {
    T result = apiRequest.get();
    if (result != null) {
      return Optional.of(result);
    }
    result = apiRequest.get();
    if (result != null) {
      return Optional.of(result);
    }
    return Optional.empty();
}

The test that I am trying to run looks like this

  @Test
  void sendEmailShouldThrowIllegalStateException() {

    when(apiclient.post(any())
        .thenReturn(null, null);
    
    assertThrows(IllegalStateException.class,
        () -> serviceClient
        .sendEndpointRegistrationEmail());
  }

I thought this would work because the mock method should return null which would trigger the exception, but it throws

org.opentest4j.AssertionFailedError: Unexpected exception type thrown ==> expected: <java.lang.IllegalStateException> but was: <java.lang.NullPointerException>

Just to clarify the issue isn't that I'm getting a NPE. The issue is that the wrong exception is being thrown.

Actually the reported exception, NullPointerException is quite natural.

Here down what is happening through your code:

• You instructed the apiclient mock to return null whenever the #post method is called
• Within your test fixture, you called serviceClient#sendEndpointRegistrationEmail with serviceClient (already having the apiclient mock injected I guess)
• When the above method is called, it will result in a call to retry(Supplier<T> apiRequest) with apiRequest argument being null (the result of your mocked call)
• T result = apiRequest.get(); would then result in a NPE as apiRequest is null

The solution would then be to return a mock for any apiclient.post(any()) call and configure the later to return null when Supplier<HttpStatus>#get is called:

<!-- language: lang-java -->

@Test
public void sendEmailShouldThrowIllegalStateException() {

  // create a Supplier mock for your request result
  Supplier apiRequestMock = Mockito.mock(Supplier.class);
  when(apiRequestMock.get()).thenReturn(null);

  // use the Supplier mock as return result
  when(apiclient.post(any())).thenReturn(apiRequestMock);

  assertThrows(IllegalStateException.class,
      () -> serviceClient
      .sendEndpointRegistrationEmail());
}