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);

Answer 1

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>
}

Answer 2

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

Answer 3

在 build.gradle 文件中将 retrofit2 版本更改为最新版本

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