Welcome to our lesson on using Fakes as test doubles in Test Driven Development (TDD) with Kotlin, JUnit, and Mockito. In this lesson, you'll explore how fakes can streamline your testing by simulating real-world components. Our journey so far has exposed you to various test doubles like dummies, stubs, spies, and mocks. Now, we'll dive into fakes, which enable you to create realistic implementations that mirror complex dependencies, making your tests more robust and reliable. As always, we'll practice the TDD cycle: Red, Green, Refactor, as we see how fakes fit into our testing strategy.

Let's see how to implement a simple fake: an InMemoryUserRepository. This serves as a stand-in for a real database repository, providing controlled behavior for our tests.

Create a class InMemoryUserRepository.kt:

Explanation:

• We create an in-memory store for users using a mutable map.
• Each function simulates typical database operations such as creating and finding users.
• The clear method ensures data isolation between tests, a crucial feature for repeatable outcomes.

By having a controlled data store, we make sure our tests are focused on business logic and not dependent on an external database. Fakes often mimic the behavior of real components, providing a safe and predictable testing environment without the complexity of external systems.

Next, we will use the fake repository to test a UserService.

1 - Red: Write Failing Tests

Create a test file UserServiceTest.kt:

Run this test to confirm it fails, as we haven't implemented the logic yet.

2 - Green: Implement Minimal Code

Now, modify the UserService.kt to ensure the test passes:

Rerun the test. It should now pass, confirming that our implementation meets the defined requirement.

In this lesson, we explored the implementation and use of fakes in TDD, specifically via an in-memory repository for user management. Remember the steps of TDD:

• Red: Write a test that fails first, setting clear goals for implementation.
• Green: Implement just enough code to make your test pass.
• Refactor: Improve code quality without altering functionality.

Leverage the practice exercises to reinforce these concepts with hands-on examples. Congratulations on navigating the complexities of testing with fakes; your commitment is paving the way for building efficient, scalable applications. This is the final lesson of the course, so kudos for reaching this milestone! Keep exploring and applying TDD principles in your projects.