"Caused by: java.lang.IllegalArgumentException: Unable to create call adapter error from retrofit2

Issue

I'm attempting to use retrofit2 for the first time. I read online many tutorials and read the docs. I understand the steps and didn't expect any errors ;) but when I run the app, I get the following error :

Caused by: java.lang.IllegalArgumentException: Unable to create call adapter for 
java.util.List<com.example.myretrofit.MarsPhoto>                                                                                                
for method MarsApiService.getPhotos

I used the create app with noActivity so my code is the basic code but added retrofit on top of the file. I just wanted to try retrofit and initiating a get call to an api.

Here is the code MainActivity.kt using kotlinx.serilization for json and retrofit2 for http get:

package com.example.myretrofit

import android.os.Bundle
import android.util.Log
import androidx.activity.ComponentActivity
import androidx.activity.compose.setContent
import androidx.compose.foundation.layout.fillMaxSize
import androidx.compose.material3.MaterialTheme
import androidx.compose.material3.Surface
import androidx.compose.material3.Text
import androidx.compose.runtime.Composable
import androidx.compose.ui.Modifier
import androidx.compose.ui.tooling.preview.Preview
import com.example.myretrofit.ui.theme.MyRetrofitTheme

//retrofit implementation
import com.jakewharton.retrofit2.converter.kotlinx.serialization.asConverterFactory
import kotlinx.serialization.json.Json
import okhttp3.MediaType.Companion.toMediaType
import retrofit2.Retrofit
import retrofit2.http.GET
import kotlinx.serialization.SerialName
import kotlinx.serialization.Serializable
import retrofit2.Call
import kotlinx.serialization.encodeToString

/**
 * This data class defines a Mars photo which includes an ID, and the image URL.
 */
@Serializable
data class MarsPhoto(
    val id: String,
    @SerialName(value = "img_src")
    val imgSrc: String
)

private const val BASE_URL =
    "https://android-kotlin-fun-mars-server.appspot.com"

/**
 * Use the Retrofit builder to build a retrofit object using a kotlinx.serialization converter
 */
private val retrofit = Retrofit.Builder()
    .addConverterFactory(Json.asConverterFactory("application/json".toMediaType()))
    .baseUrl(BASE_URL)
    .build()

/**
 * Retrofit service object for creating api calls
 */
interface MarsApiService {
    @GET("photos")
    fun getPhotos(): List<MarsPhoto>
}

/**
 * A public Api object that exposes the lazy-initialized Retrofit service
 */
object MarsApi {
    val retrofitService: MarsApiService by lazy {
        retrofit.create(MarsApiService::class.java)
    }
}


//end of retrofit implementation

val listResult = MarsApi.retrofitService.getPhotos()

//                    for (item in listResult) {
//                        Log.i("ServerItem : ", "$item")
//                    }


class MainActivity : ComponentActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContent {
            MyRetrofitTheme {
                // A surface container using the 'background' color from the theme
                Surface(
                    modifier = Modifier.fillMaxSize(),
                    color = MaterialTheme.colorScheme.background
                ) {

                    val listResult = MarsApi.retrofitService.getPhotos()

                    Log.i("ServerItem : ", "print something")
                    Greeting("Android")
                }
            }
        }
    }
}

@Composable
fun Greeting(name: String, modifier: Modifier = Modifier) {
    Text(
        text = "Hello $name!",
        modifier = modifier
    )
}

@Preview(showBackground = true)
@Composable
fun GreetingPreview() {
    MyRetrofitTheme {
        Greeting("Android")
    }
}

Clearly I missed something while reading all the tutorials and docs for retrofit2. The error indicates that the interface code is wrong but I don't know why? I don't have an adapter and I actually don't need an adapter with kotlinx.serilization, right??

How to fix this error so I can get the response from the api?

The docs is here

Please keep in mind that I'm at level 0 in android studio/kotlin/apps development. I just started three weeks ago. I understand and able to use compose well. I understand how to work with activities and coroutines. I don't want to add coRoutines here because I wanted to understand retrofit2 so I'm ok with the call blocking the thread.

I believe the error is because the interface is fun getPhotos() when I take it out, the error goes away, the app runs correctly but I get nothing from the retrofit instance. No logging of any type.

Thank you for trying to help.

How to fix this error so I can get the response from the api?

Solution

kotlinx.serialization is not officially available as a converter factory for Retrofit. If you still want to use it with Retrofit you need to use this library.

However, I would suggest an easier option like GsonConverter which is available for Retrofit. To do this, first, add the dependency:

implementation("com.squareup.retrofit2:converter-gson:<your_retrofit_version>")

And then, add the converter factory when creating the Retrofit client:

Retrofit.Builder()  
    .baseUrl("<your_api_base_url>")
    .addConverterFactory(GsonConverterFactory.create())
    .build()

For a more detailed usecase in a project with dependency injection check out this.

Answered By - Atick Faisal