Your querylist is declared, but not initialized. It is bad practice to @Mock the whole list - See this question and answers. In the snippets provided the @Mock annotation doesn't even seem necessary.

There are a couple of options, the first seems most applicable to your situation:

• Remove the @Mock annotation, initialize the list List<UserLoginOutput> queryList = new ArrayList<>(); and add the expectedOutput directly to the list with queryList.add(expectedOutput);. Mocking the call to size() is no longer necessary.
• initialize the list populated if you have static (or predetermined) values: List<UserLoginOutput> queryList = Arrays.asList(mockedUserLoginOutput); This would remove the need to mock the call to size() and the @Mock annotation.
• Initialize as an empty list but don't add the expectedOutput to the list and keep mocking the call to size. You might run into other issues as described in the linked article if other tests use Collection functions like iterator().
• If the list, for whatever reason, absolutely has to be mocked you will need to set it explicitly to the class under test, as is done in this answer.