AbstractMethodError when using RxJavaCallAdapterFactory on Retrofit 2

Question

I get this error:

FATAL EXCEPTION: main
E/AndroidRuntime: java.lang.AbstractMethodError: abstract method not implemented
at retrofit.RxJavaCallAdapterFactory.get(RxJavaCallAdapterFactory.java)
at retrofit.Retrofit.nextCallAdapter(Retrofit.java:189)
at retrofit.Retrofit.callAdapter(Retrofit.java:175)
at retrofit.MethodHandler.createCallAdapter(MethodHandler.java:45)
at retrofit.MethodHandler.create(MethodHandler.java:26)
at retrofit.Retrofit.loadMethodHandler(Retrofit.java:151)
at retrofit.Retrofit$1.invoke(Retrofit.java:132)
at $Proxy0.getPosts(Native Method)

when trying to use RxJavaCallAdapterFactory on retrofit. I'm using com.squareup.retrofit:retrofit:2.0.0-beta1 and com.squareup.retrofit:adapter-rxjava:2.0.0-beta1.

Here's how I created the api interface:

Retrofit retrofit = new Retrofit.Builder()
        .baseUrl(FORUM_SERVER_URL)
        .addCallAdapterFactory(RxJavaCallAdapterFactory.create())
        .addConverterFactory(GsonConverterFactory.create())
        .build();

mForumApi = retrofit.create(ForumApi.class);

The FORUM_SERVER_URL is
private static final String FORUM_SERVER_URL = "http://jsonplaceholder.typicode.com";

my interface method is:

@GET("/posts")
public Observable<List<Post>> getPosts();

I call it via:

   mForum.getApi()
            .getPosts()
            .subscribeOn(Schedulers.newThread())
            .observeOn(AndroidSchedulers.mainThread())
            .subscribe(new Observer<List<Post>>() {
                @Override
                public void onCompleted() {}
                @Override
                public void onError(Throwable e) {}
                @Override
                public void onNext(List<Post> posts) {
                    mView.displayPosts(posts);
                }
            });
}

getApi returns mForumApi
getPosts is where the error happens, it's the API call

Answer 1

For me it turned out that I was using different beta versions of the components

Changing (notice beta1):

compile 'com.squareup.retrofit:converter-simplexml:2.0.0-beta2'
compile 'com.squareup.retrofit:adapter-rxjava:2.0.0-beta1'

to (now beta2)

compile 'com.squareup.retrofit:converter-simplexml:2.0.0-beta2'
compile 'com.squareup.retrofit:adapter-rxjava:2.0.0-beta2'

made it work for me.

Stupid error but yeah...