Code Smell 254 - Mystery Guest

You assert that something happened, but why?

Maximiliano Contieri

Jun 03, 2024

TL;DR: Be explicit when creating tests to ensure clarity and maintainability

Problems

Readability
Coupling to external databases, global state or singletons, static methods or external services
Maintenance Difficulty
Debugging Complexity
Hidden Dependencies

Solutions

Be Explicit
Inline the setup
Use dependency Injection
Use mocking with caution

Context

Your test depends on external data or configurations not immediately visible within the test itself.

This obscures the test’s setup, making it difficult for someone reading it to understand what is being tested and why it might fail.

Every test case should have three stages:

Setup: Initialize and configure everything needed for the test.
Excercise: Execute the code being tested.
Assert: Verify the expected outcome.

All of them must be explicit

Sample Code

Wrong

@Test
void shouldReturnAnswerWhenAnswerExists() {
    User answer = KnowledgeRepository.findAnswerToQuestion(42);
    assertNotNull(answer);
}

Right

@Test
void shouldReturnAnswerWhenAnswerExists() {
    KnowledgeRepository knowledgeRepository = 
        new InMemoryKnowledgeRepository();
    Answer expectedAnswer = new Answer(42, "The Ultimate");
    knowledgeRepository.save(expectedAnswer);

    Answer actualAnswer = answerRepository.findAnswerToQuestion(42);
    assertNotNull(actualAnswer);
    assertEquals(expectedAnswer, actualAnswer);
}