Retrofit setup in Android Kotlin - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is Retrofit in Android development?

Retrofit is a type-safe HTTP client for Android and Java. It helps you easily connect your app to web services by turning HTTP API into Kotlin interfaces.

beginner

Which dependency do you add to your build.gradle to use Retrofit?

You add implementation 'com.squareup.retrofit2:retrofit:2.9.0' to your app's build.gradle file to include Retrofit.

intermediate

How do you create a Retrofit instance in Kotlin?

Use Retrofit.Builder() to set the base URL and converter, then call build(). For example:
val retrofit = Retrofit.Builder() .baseUrl("https://api.example.com/") .addConverterFactory(GsonConverterFactory.create()) .build()

intermediate

What is the role of a ConverterFactory in Retrofit?

ConverterFactory converts JSON or other data formats from the web service into Kotlin objects and vice versa. GsonConverterFactory is commonly used for JSON.

intermediate

How do you define an API interface for Retrofit?

Create a Kotlin interface with functions annotated with HTTP methods like @GET or @POST. Each function returns a Call or suspend function for coroutines. Example:
interface ApiService { @GET("users") suspend fun getUsers(): List<User> }

What is the first step to use Retrofit in your Android app?

A Add Retrofit dependency to build.gradle

B Create an API interface

C Call the API directly

D Write XML layout

You must first add the Retrofit library dependency to your project before using it.

Which Retrofit component sets the base URL for API calls?

A Retrofit.Builder().baseUrl()

B ConverterFactory

C API interface

D Call object

The baseUrl() method in Retrofit.Builder sets the root URL for all API requests.

What does GsonConverterFactory do in Retrofit?

A Manages API keys

B Handles network errors

C Converts JSON to Kotlin objects

D Creates UI layouts

GsonConverterFactory converts JSON responses into Kotlin data classes and vice versa.

How do you declare a GET request in a Retrofit API interface?

A In AndroidManifest.xml

B Using a JSON file

C Inside build.gradle

D @GET annotation on a function

You use the @GET annotation on a function inside the API interface to declare a GET request.

Which return type is commonly used for Retrofit API calls with Kotlin coroutines?

A Void

B suspend function returning data

C String

D Int

Using suspend functions allows Retrofit to work smoothly with Kotlin coroutines for asynchronous calls.

Explain the steps to set up Retrofit in an Android Kotlin project.

Think about what you need to add, build, and define before making API calls.

Describe how Retrofit converts JSON responses into Kotlin objects.

Focus on the part that changes raw data into usable app data.

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