Welcome to our lesson on using Fakes as test doubles in Test Driven Development (TDD) with Scala. 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.scala:

class InMemoryUserRepository:

  private val users = scala.collection.concurrent.TrieMap[String, User]()
  private var currentId: Int = 1

  private def generateId(): String =
    val newId = currentId
    currentId += 1
    newId.toString

  def create(userData: User): User =
    val user = User()
    user.setId(generateId())
    user.setEmail(userData.getEmail)
    user.setName(userData.getName)
    user.setCreatedAt(java.time.LocalDateTime.now())
    users.put(user.getId, user)
    user

  def findById(id: String): User | Null =
    users.get(id).getOrElse(null)

  def findByEmail(email: String): User | Null =
    users.values.find(u => u.getEmail.equalsIgnoreCase(email)).getOrElse(null)

  def update(id: String, data: User): User | Null =
    val existing = users.get(id)
    if existing.isEmpty then
      null
    else
      val user = existing.get
      if data.getEmail != null then user.setEmail(data.getEmail)
      if data.getName != null then user.setName(data.getName)
      users.put(id, user)
      user

  def delete(id: String): Boolean =
    users.remove(id).isDefined

  def findAll(): List[User] =
    users.values.toList

  def clear(): Unit =
    users.clear()
    currentId = 1
end InMemoryUserRepository

Explanation:

• We create an in-memory store for users using a concurrent TrieMap.
• 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.