Repository testing with fakes in Android Kotlin - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a fake repository class implementing the UserRepository interface.

Android Kotlin

class FakeUserRepository : UserRepository {
    override fun getUser(id: Int): User {
        return [1]
    }
}

Drag options to blanks, or click blank then click option'

AUser()

Bnull

Cthrow Exception("Not implemented")

DUser(id, "Fake User")

Attempts:

3 left

💡 Hint

Common Mistakes

Returning null instead of a User object.

Throwing an exception instead of returning a fake user.

The fake repository returns a dummy User object with the given id and a fixed name.

2fill in blank

medium

Complete the test function to verify the fake repository returns the expected user name.

Android Kotlin

fun testGetUserReturnsFake() {
    val repo = FakeUserRepository()
    val user = repo.getUser(1)
    assert(user.name == [1])
}

Drag options to blanks, or click blank then click option'

A"Real User"

B"Test User"

C"Fake User"

D"Unknown"

Attempts:

3 left

💡 Hint

Common Mistakes

Using the wrong expected name in the assertion.

Confusing the fake user name with a real user name.

The fake repository returns a user with the name "Fake User" as defined in the fake.

3fill in blank

hard

Fix the error in the fake repository to store users in a mutable map and return them by id.

Android Kotlin

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

    fun addUser(user: User) {
        users[[1]] = user
    }

    override fun getUser(id: Int): User? {
        return users[[2]]
    }
}

Drag options to blanks, or click blank then click option'

Auser.id

Buser.name

Cid

Duser

Attempts:

3 left

💡 Hint

Common Mistakes

Using user.name as the key instead of user.id.

Using user instead of id to retrieve from the map.

The addUser function uses user.id as the key to store the user. The getUser function uses the id parameter to retrieve the user.

4fill in blank

hard

Fill both blanks to complete the test that adds a user to the fake repository and verifies retrieval.

Android Kotlin

fun testAddAndGetUser() {
    val repo = FakeUserRepository()
    val user = User(10, "Test User")
    repo.[1](user)
    val retrieved = repo.[2](10)
    assert(retrieved?.name == "Test User")
}

Drag options to blanks, or click blank then click option'

AaddUser

BgetUser

CremoveUser

DupdateUser

Attempts:

3 left

💡 Hint

Common Mistakes

Using removeUser or updateUser instead of addUser or getUser.

Mixing up the order of method calls.

The test calls addUser to add the user, then getUser to retrieve it by id.

5fill in blank

hard

Fill all three blanks to create a fake repository with a mutable list and implement add, get, and clear methods.

Android Kotlin

class FakeUserRepository : UserRepository {
    private val users = mutableListOf<User>()

    fun [1](user: User) {
        users.[2](user)
    }

    fun [3]() {
        users.clear()
    }

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

Drag options to blanks, or click blank then click option'

AaddUser

Badd

CclearUsers

Dclear

Attempts:

3 left

💡 Hint

Common Mistakes

Using incorrect method names that don't match the calls.

Calling wrong list methods like remove instead of add.

The method to add a user is named addUser, which calls the list's add method. The method to clear users is named clear, which calls users.clear().

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