Define a Retrofit API Interface
Declare endpoints with annotations.
Define a Retrofit API Interface is a free Jetpack Compose Academy lesson on CoddyKit — lesson 1 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the Jetpack Compose Academy learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.
What Retrofit Does
Retrofit turns a plain Kotlin interface into a working HTTP client, so you describe your API and it writes the networking code for you.
It Starts With an Interface
You declare each endpoint as a function inside a Kotlin interface. Retrofit reads the annotations and builds the real implementation at runtime.
interface UserApi {
// endpoint functions go here
}Mark a GET Request
The @GET annotation tells Retrofit this function fetches data. You pass the path that sits after your base URL. 🌐
@GET("users")
fun getUsers(): Call<List<User>>POST, PUT and DELETE Too
Beyond GET, Retrofit offers @POST, @PUT, and @DELETE so one interface can read and write data on your server.
@POST("users")
fun createUser(@Body user: User): Call<User>Path Parameters
Use @Path to inject a value into the URL. Retrofit swaps the named placeholder in braces for the argument you pass in.
@GET("users/{id}")
fun getUser(@Path("id") id: Int): Call<User>Query Parameters
Add @Query for values after the question mark, like search terms or page numbers. Retrofit appends them to the URL for you.
@GET("search")
fun search(@Query("q") term: String): Call<Result>The Return Type
The function's return type tells Retrofit what shape to expect back. A data class here maps straight onto the JSON response.
@GET("users/{id}")
fun getUser(@Path("id") id: Int): Call<User>Set the Base URL
You build one Retrofit instance with the base URL. Every endpoint path you declared is joined onto it automatically.
val retrofit = Retrofit.Builder()
.baseUrl("https://api.example.com/")
.build()Create the Implementation
Call create with your interface class and Retrofit returns a ready-to-use object. You never write the request code yourself.
val api = retrofit.create(UserApi::class.java)Add a Converter Factory
A converter factory teaches Retrofit how to turn JSON into your data classes. You add it once when building the instance.
.addConverterFactory(MoshiConverterFactory.create())Don't Forget the Permission
To reach the network, declare the INTERNET permission in your manifest. Without it, every request fails before it even starts.
<uses-permission android:name="android.permission.INTERNET" />Quick Check
Which annotation injects a value directly into the URL path?
Recap: Your API Interface
You described endpoints as annotated interface functions, set a base URL, added a converter, and let Retrofit build the client. Next you'll parse the JSON it returns.
Frequently asked questions
Is the “Define a Retrofit API Interface” lesson free?
Yes — the full text of “Define a Retrofit API Interface” is free to read here on the web, and the Jetpack Compose Academy course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the Jetpack Compose Academy course, upgrade to CoddyKit PRO.
What will I learn in “Define a Retrofit API Interface”?
Declare endpoints with annotations. You practise Jetpack Compose Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.
Do I need any experience to start Jetpack Compose Academy?
No prior experience is required. Jetpack Compose Academy on CoddyKit is structured for beginners through advanced learners; this is — lesson 1 of 4, so you can start here or from the beginning and move at your own pace.
How long does the “Define a Retrofit API Interface” lesson take?
Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.
Can I write and run code in this Jetpack Compose Academy lesson?
Yes. Every Jetpack Compose Academy lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.
All lessons in this course
- Define a Retrofit API Interface
- Parsing JSON with Moshi
- suspend Calls in the ViewModel
- Loading, Success & Error UI