Retrofit 2.0: Unable to create call adapter for class

Question

I'm using Retrofit 2.0-beta1. How can I make a simple synchronous request and deserialize a JSON response as a POJO list? This is my interface:

public interface ArticlesAPI {
    @GET("/articles")
    List<ArticleResource> getArticles();
}

and

public class ArticleResource {

    public String id;
    public String name;

    public Article toArticle() {
        return new Article(id, name);
    }
}

but I get this error:

>  Unable to create call adapter for
> java.util.List<com.bla.bla.rest.resources.ArticleResource>

Retrofit build

Retrofit retrofit = (new Retrofit.Builder()).baseUrl(this.baseUrl).build();
ArticlesAPI api = retrofit.create(ArticlesAPI.class);

Solution 1

Accordingly to the code you posted you have to change from

List<ArticleResource> getArticles();

to

Call<List<ArticleResource>> getArticles();

You might also want to call explicitly addConverterFactory to set the converter you want to use

Solution 2

This issue can also occur if you are using kotlin coroutines and the function is not suspend function

public interface ArticlesAPI {
 @GET("url.../habits/progress/")
    suspend fun getHabitsTasks(): BaseResponse<HabitResModel>
}

Solution 3

Change the retrofit2 version to latest in build.gradle file

implementation "com.squareup.retrofit2:retrofit:2.9.0"

Comments

[Radu]

Can you please explain why that is?

[Blackbelt]

@Radu why that is what ?

[Radu]

Why has Retrofit 2.0 decided to wrap everything in a Call<T>. I think it's to encapsulate the sync/async behavior?

[Blackbelt]

@Radu, it is highly possibile. You should ask square to be 100% sure

[gbruscatto]

Ka-chow! I forgot the suspend prefix. Thank you! :)

[Ritzor]

Ops..why android? why?

[user10997009]

Hello @Rahul,I am new with MVVM and corotuines,i got the same problem.Can you help me.Here is my code github.com/diptoroy/RetrofitKotlin

[Nouman Ghaffar]

saved the day...