Retrofit setup in Android Kotlin - Practice Problems & Coding Challenges

Challenge - 5 Problems

📝 Syntax

intermediate

Correct Retrofit Builder Syntax

Which option correctly creates a Retrofit instance with base URL "https://api.example.com/" and Gson converter?

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

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

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

💡 Hint

Remember to call the create() method with parentheses and include the trailing slash in the base URL.

Option D correctly calls GsonConverterFactory.create() with parentheses and includes the trailing slash in the base URL. Option D misses parentheses on create(), Option D misses the .create() call and trailing slash, and Option D misses parentheses on build().

❓ ui_behavior

intermediate

Retrofit Network Call Behavior

What happens when you call a Retrofit service method that returns Call and you execute it asynchronously with enqueue()?

A The network call runs on a background thread and the callback methods onResponse or onFailure are called on the main thread.

B The network call runs on the main thread and blocks the UI until it finishes.

C The network call runs on a background thread but the callback methods are called on a background thread.

D The network call runs synchronously and returns the response immediately.

💡 Hint

Think about how Retrofit handles threading for asynchronous calls.

Retrofit's enqueue() method runs the network call on a background thread and delivers the callback results on the main thread, so UI updates can be done safely.

❓ lifecycle

advanced

Handling Retrofit Call Cancellation

Which option correctly cancels an ongoing Retrofit Call to avoid memory leaks when an Android Activity is destroyed?

Android Kotlin

var call: Call<ResponseBody>? = null

fun fetchData() {
  call = retrofitService.getData()
  call?.enqueue(object : Callback<ResponseBody> {
    override fun onResponse(call: Call<ResponseBody>, response: Response<ResponseBody>) {
      // handle response
    }
    override fun onFailure(call: Call<ResponseBody>, t: Throwable) {
      // handle failure
    }
  })
}

fun onDestroy() {
  // What to do here?
}

A call = null

B call?.cancel()

C call?.clone()

D call?.enqueue(null)

💡 Hint

To stop a network call, you must explicitly cancel it.

Calling cancel() on the Call object stops the network request and prevents callbacks from being invoked, avoiding leaks. Setting call to null does not stop the request. clone() creates a new call. enqueue(null) is invalid.

❓ navigation

advanced

Retrofit and Navigation Timing

You want to navigate to a new screen only after a successful Retrofit network response. Which approach ensures navigation happens at the right time?

A Call navigation code before enqueue()

B Call navigation code immediately after enqueue() without waiting

C Call navigation code inside onResponse callback after checking response.isSuccessful

D Call navigation code inside onFailure callback

💡 Hint

When does Retrofit notify you of a successful response?

Navigation should happen inside onResponse after confirming the response is successful. Calling navigation immediately after enqueue() runs before the network call finishes. onFailure means the call failed. Before enqueue() means no network call started yet.

🧠 Conceptual

expert

Retrofit Converter Factory Role

What is the main role of a Converter Factory like GsonConverterFactory in Retrofit setup?

A To convert JSON response data into Kotlin/Java objects and vice versa for requests

B To manage network connection retries automatically

C To handle UI thread synchronization for network calls

D To cache network responses locally on the device

💡 Hint

Think about how Retrofit transforms raw data into usable objects.

Converter factories like GsonConverterFactory parse JSON strings into Kotlin/Java objects and convert objects into JSON for sending requests. They do not manage retries, threading, or caching.