Retrofit setup in Android Kotlin - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a Retrofit instance with the base URL.

Android Kotlin

val retrofit = Retrofit.Builder()
    .baseUrl("[1]")
    .build()

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

A"api.example.com"

B"https://api.example.com/"

C"http://api.example.com"

D"https://example.com/api"

Attempts:

3 left

💡 Hint

Common Mistakes

Omitting the scheme (http/https) causes errors.

Forgetting the trailing slash at the end of the URL.

The base URL must end with a slash and include the scheme (https://) for Retrofit to work correctly.

2fill in blank

medium

Complete the code to add a JSON converter factory to Retrofit.

Android Kotlin

val retrofit = Retrofit.Builder()
    .baseUrl("https://api.example.com/")
    .addConverterFactory([1].create())
    .build()

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

AGsonConverterFactory

BJsonConverterFactory

CMoshiConverterFactory

DXmlConverterFactory

Attempts:

3 left

💡 Hint

Common Mistakes

Using a converter factory that does not match the data format.

Forgetting to add the converter factory causes parsing errors.

GsonConverterFactory is commonly used to convert JSON data to Kotlin objects in Retrofit.

3fill in blank

hard

Fix the error in the Retrofit builder code by completing the blank.

Android Kotlin

val retrofit = Retrofit.Builder()
    .baseUrl("https://api.example.com/")
    .addConverterFactory(GsonConverterFactory.[1]())
    .build()

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

AgetInstance

Bnew

Cbuild

Dcreate

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'new' keyword which is invalid in Kotlin.

Using 'build' or 'getInstance' which do not exist for this class.

The correct method to get a GsonConverterFactory instance is 'create()'.

4fill in blank

hard

Fill both blanks to define a Retrofit service interface with a GET request.

Android Kotlin

interface ApiService {
    @[1]("users")
    fun getUsers(): [2]<List<User>>
}

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

AGET

BPOST

CCall

DResponse

Attempts:

3 left

💡 Hint

Common Mistakes

Using POST instead of GET for fetching data.

Using Response instead of Call as the return type.

The GET annotation defines a GET request, and Call wraps the response type for Retrofit.

5fill in blank

hard

Fill all three blanks to build a Retrofit instance with base URL, JSON converter, and create the service.

Android Kotlin

val retrofit = Retrofit.Builder()
    .baseUrl([1])
    .addConverterFactory([2].create())
    .build()

val service = retrofit.[3](ApiService::class.java)

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

A"https://api.example.com/"

BGsonConverterFactory

Ccreate

DcreateService

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'createService' instead of 'create' method on retrofit.

Missing quotes around the base URL.

Using wrong converter factory.

Use the correct base URL string, GsonConverterFactory with create(), and Retrofit's create() method to get the service instance.

Practice

1. What is the main purpose of Retrofit in Android Kotlin development?

easy

A. To manage local databases

B. To simplify network calls and convert web data into Kotlin objects

C. To create user interfaces easily

D. To handle app navigation between screens

Solution

1. Step 1: Understand Retrofit's role

   Retrofit is a library designed to make network calls easier and convert JSON or XML responses into Kotlin objects automatically.

2. Step 2: Compare with other options

   Options A, B, and D relate to UI, databases, and navigation, which are not Retrofit's purpose.

3. Final Answer:

   To simplify network calls and convert web data into Kotlin objects -> Option B

4. Quick Check:

   Retrofit = Network calls + data conversion [OK]

Hint: Retrofit = network calls + data conversion [OK]

Common Mistakes:

• Confusing Retrofit with UI or database libraries
• Thinking Retrofit manages navigation
• Assuming Retrofit is for local storage

2. Which of the following is the correct way to start building a Retrofit instance in Kotlin?

easy

A. val retrofit = Retrofit.newInstance("https://api.example.com")

B. val retrofit = Retrofit.create().baseUrl("https://api.example.com")

C. val retrofit = Retrofit.Builder().baseUrl("https://api.example.com").build()

D. val retrofit = Retrofit.Builder().create("https://api.example.com")

Solution

1. Step 1: Recall Retrofit Builder syntax

   The correct way is to use Retrofit.Builder(), set the baseUrl, and then call build() to create the instance.

2. Step 2: Check each option

   val retrofit = Retrofit.Builder().baseUrl("https://api.example.com").build() uses the correct Builder pattern. Options A, B, and C use incorrect methods or order.

3. Final Answer:

   val retrofit = Retrofit.Builder().baseUrl("https://api.example.com").build() -> Option C

4. Quick Check:

   Builder + baseUrl + build() = correct setup [OK]

Hint: Use Builder, set baseUrl, then build() [OK]

Common Mistakes:

• Calling create() before build()
• Using newInstance() which doesn't exist
• Passing URL to create() instead of baseUrl()

3. Given this Retrofit setup code snippet, what will be the output if you call service.getUsers() and the server returns JSON data?

val retrofit = Retrofit.Builder()
  .baseUrl("https://api.example.com/")
  .addConverterFactory(GsonConverterFactory.create())
  .build()

interface ApiService {
  @GET("users")
  suspend fun getUsers(): List<User>
}

val service = retrofit.create(ApiService::class.java)

medium

A. A raw JSON string from the server

B. A null value because the call is not executed

C. An error because GsonConverterFactory is missing

D. A List of User objects parsed from the JSON response

Solution

1. Step 1: Understand the converter role

   GsonConverterFactory converts JSON responses into Kotlin objects automatically.

2. Step 2: Analyze the service call

   Calling getUsers() returns a List<User> parsed from JSON because of the converter and suspend function.

3. Final Answer:

   A List of User objects parsed from the JSON response -> Option D

4. Quick Check:

   GsonConverterFactory + suspend fun = parsed objects [OK]

Hint: GsonConverterFactory parses JSON to objects [OK]

Common Mistakes:

• Expecting raw JSON instead of parsed objects
• Forgetting to add converter factory
• Not understanding suspend functions

4. Identify the error in this Retrofit setup code:

val retrofit = Retrofit.Builder()
  .baseUrl("https://api.example.com")
  .addConverterFactory(GsonConverterFactory)
  .build()

medium

A. Missing parentheses after GsonConverterFactory

B. baseUrl should not have a trailing slash

C. Retrofit.Builder() should be Retrofit.newBuilder()

D. build() should be called before addConverterFactory

Solution

1. Step 1: Check addConverterFactory usage

   GsonConverterFactory is a class; it requires calling create() method or parentheses to instantiate.

2. Step 2: Verify other parts

   baseUrl can have or not have trailing slash; Retrofit.Builder() is correct; build() must be last.

3. Final Answer:

   Missing parentheses after GsonConverterFactory -> Option A

4. Quick Check:

   addConverterFactory needs instance, not class [OK]

Hint: Call create() on converter factory class [OK]

Common Mistakes:

• Passing class instead of instance to addConverterFactory
• Confusing baseUrl format
• Wrong order of builder methods

5. You want to create a Retrofit instance that supports both JSON and XML responses. Which setup correctly adds both converters?

hard

A. Retrofit.Builder().baseUrl("https://api.example.com/").addConverterFactory(SimpleXmlConverterFactory.create()).addConverterFactory(GsonConverterFactory.create()).build()

B. Retrofit.Builder().baseUrl("https://api.example.com/").addConverterFactory(GsonConverterFactory.create()).addConverterFactory(SimpleXmlConverterFactory).build()

C. Retrofit.Builder().baseUrl("https://api.example.com/").addConverterFactory(GsonConverterFactory).addConverterFactory(SimpleXmlConverterFactory).build()

D. Retrofit.Builder().baseUrl("https://api.example.com/").addConverterFactory(GsonConverterFactory.create()).build().addConverterFactory(SimpleXmlConverterFactory.create())

Solution

1. Step 1: Understand converter order importance

   Retrofit tries converters in order added. XML converter should come first to handle XML, then JSON.

2. Step 2: Check method chaining and instantiation

   Converters must be instantiated with create(). Retrofit.Builder().baseUrl("https://api.example.com/").addConverterFactory(SimpleXmlConverterFactory.create()).addConverterFactory(GsonConverterFactory.create()).build() adds XML first, then JSON, correctly.

3. Step 3: Eliminate wrong options

   Retrofit.Builder().baseUrl("https://api.example.com/").addConverterFactory(GsonConverterFactory.create()).addConverterFactory(SimpleXmlConverterFactory.create()).build() reverses order; C misses create() calls; D calls addConverterFactory after build(), which is invalid.

4. Final Answer:

   Retrofit.Builder().baseUrl("https://api.example.com/").addConverterFactory(SimpleXmlConverterFactory.create()).addConverterFactory(GsonConverterFactory.create()).build() -> Option A

5. Quick Check:

   Converters order matters; XML first, then JSON [OK]

Hint: Add XML converter before JSON converter [OK]

Common Mistakes:

• Wrong order of converters causing parsing errors
• Not calling create() on converters
• Adding converters after build() call