Repository testing with fakes in Android Kotlin

Introduction

We test repositories with fakes to check if our data handling works correctly without using real databases or network calls.

When you want to test data fetching logic without connecting to a real server.

When you want to check how your app handles data storage without using a real database.

When you want fast tests that don't depend on external systems.

When you want to isolate repository logic from other parts of the app.

When you want to simulate different data scenarios easily.

Syntax

Android Kotlin

interface UserRepository {
    suspend fun getUser(id: String): User?
}

class FakeUserRepository : UserRepository {
    private val users = mutableMapOf<String, User>()

    fun addUser(user: User) {
        users[user.id] = user
    }

    override suspend fun getUser(id: String): User? {
        return users[id]
    }
}

A FakeUserRepository implements the same interface as the real repository.

It stores data in memory for quick testing without real data sources.

Examples

This fake repository stores products in memory and returns them when requested.

Android Kotlin

interface ProductRepository {
    suspend fun getProduct(id: String): Product?
}

class FakeProductRepository : ProductRepository {
    private val products = mutableMapOf<String, Product>()

    fun addProduct(product: Product) {
        products[product.id] = product
    }

    override suspend fun getProduct(id: String): Product? {
        return products[id]
    }
}

This version uses a fixed list of users instead of a mutable map.

Android Kotlin

class FakeUserRepository : UserRepository {
    private val users = listOf(User("1", "Alice"), User("2", "Bob"))

    override suspend fun getUser(id: String): User? {
        return users.find { it.id == id }
    }
}

Sample App

This example shows a fake user repository storing users in memory. The test function adds users and fetches one by ID, printing the name.

Android Kotlin

data class User(val id: String, val name: String)

interface UserRepository {
    suspend fun getUser(id: String): User?
}

class FakeUserRepository : UserRepository {
    private val users = mutableMapOf<String, User>()

    fun addUser(user: User) {
        users[user.id] = user
    }

    override suspend fun getUser(id: String): User? {
        return users[id]
    }
}

suspend fun testFakeRepository() {
    val fakeRepo = FakeUserRepository()
    fakeRepo.addUser(User("1", "Alice"))
    fakeRepo.addUser(User("2", "Bob"))

    val user = fakeRepo.getUser("1")
    println(user?.name ?: "User not found")
}

// To run testFakeRepository, use a coroutine scope or runBlocking in main

Important Notes

Fakes are simple and fast but do not test real database or network behavior.

Use fakes to isolate repository logic from external dependencies.

Remember to keep fake data realistic for better test coverage.

Summary

Repository testing with fakes helps test data logic without real data sources.

Fakes implement the same interface as real repositories but use in-memory data.

This approach makes tests faster and more reliable by avoiding external dependencies.

Practice

1. What is the main purpose of using a fake repository in Android Kotlin testing?

easy

A. To replace the Android framework in tests

B. To improve UI animations during testing

C. To increase app size by adding more classes

D. To simulate data operations without accessing real databases or network

Solution

1. Step 1: Understand the role of a fake repository

   A fake repository mimics the real repository's behavior but uses in-memory data instead of real data sources.

2. Step 2: Identify the benefit in testing

   This allows tests to run quickly and reliably without depending on databases or network calls.

3. Final Answer:

   To simulate data operations without accessing real databases or network -> Option D

4. Quick Check:

   Fake repository = simulate data without real sources [OK]

Hint: Fakes replace real data sources to speed up tests [OK]

Common Mistakes:

• Thinking fakes improve UI animations
• Confusing fakes with Android framework replacement
• Assuming fakes increase app size

2. Which Kotlin syntax correctly declares a fake repository class implementing an interface UserRepository?

easy

A. class FakeUserRepository extends UserRepository { }

B. class FakeUserRepository implements UserRepository { }

C. class FakeUserRepository : UserRepository { }

D. class FakeUserRepository inherits UserRepository { }

Solution

1. Step 1: Recall Kotlin interface implementation syntax

   Kotlin uses a colon (:) to implement interfaces, not keywords like implements or extends.

2. Step 2: Match correct syntax

   class FakeUserRepository : UserRepository { } uses : correctly to implement UserRepository.

3. Final Answer:

   class FakeUserRepository : UserRepository { } -> Option C

4. Quick Check:

   Kotlin interface implementation uses ':' [OK]

Hint: Use ':' to implement interfaces in Kotlin [OK]

Common Mistakes:

• Using Java keywords like implements or extends in Kotlin
• Confusing inheritance with interface implementation
• Omitting the colon ':'

3. Given this fake repository code snippet, what will println(fakeRepo.getUserCount()) output?

interface UserRepository {
  fun getUserCount(): Int
}

class FakeUserRepository : UserRepository {
  private val users = mutableListOf("Alice", "Bob")
  override fun getUserCount() = users.size
}

val fakeRepo = FakeUserRepository()

medium

A. 2

B. 1

C. 0

D. Compilation error

Solution

1. Step 1: Analyze the users list initialization

   The users list contains two elements: "Alice" and "Bob".

2. Step 2: Understand getUserCount() behavior

   The method returns the size of the users list, which is 2.

3. Final Answer:

   2 -> Option A

4. Quick Check:

   users.size = 2 [OK]

Hint: Count elements in the list for size [OK]

Common Mistakes:

• Assuming empty list by default
• Confusing method return with list content
• Expecting compilation error due to interface

4. Identify the error in this fake repository implementation:

class FakeUserRepository : UserRepository {
  private val users = mutableListOf<String>()
  fun getUserCount() = users.size
}

medium

A. Missing override keyword for getUserCount()

B. users list should be immutable

C. Class should not implement UserRepository

D. No constructor defined

Solution

1. Step 1: Check interface method implementation

   The interface UserRepository requires getUserCount() to be overridden.

2. Step 2: Verify method declaration

   The method lacks the override keyword, causing a compilation error.

3. Final Answer:

   Missing override keyword for getUserCount() -> Option A

4. Quick Check:

   Interface methods need 'override' in Kotlin [OK]

Hint: Always use 'override' for interface methods [OK]

Common Mistakes:

• Forgetting 'override' keyword
• Thinking mutable list is wrong here
• Assuming constructor is mandatory

5. You want to test a repository method that fetches users filtered by minimum age. Your fake repository stores users as data class User(val name: String, val age: Int). Which fake repository method correctly returns users older than or equal to the given age?

hard

A. fun getUsersByMinAge(minAge: Int) = users.filter { it.age <= minAge }

B. fun getUsersByMinAge(minAge: Int) = users.filter { it.age >= minAge }

C. fun getUsersByMinAge(minAge: Int) = users.filter { it.age > minAge }

D. fun getUsersByMinAge(minAge: Int) = users.filter { it.name.length >= minAge }

Solution

1. Step 1: Understand filtering condition

   The requirement is to get users with age greater than or equal to minAge.

2. Step 2: Match correct filter predicate

   fun getUsersByMinAge(minAge: Int) = users.filter { it.age >= minAge } uses it.age >= minAge, which matches the requirement exactly.

3. Final Answer:

   fun getUsersByMinAge(minAge: Int) = users.filter { it.age >= minAge } -> Option B

4. Quick Check:

   Filter with '>= minAge' for correct users [OK]

Hint: Use '>=' to include minimum age users [OK]

Common Mistakes:

• Using '>' excludes users exactly at minAge
• Using '<=' returns younger users
• Filtering by name length instead of age