Unable to create call adapter for io.reactivex.Observable

I'm going to send a simple get method to my server(it is Rails app) and get the result using RxJava and Retrofit. The thing that I did is:

My interface:

public interface ApiCall {
    String SERVICE_ENDPOINT = "https://198.50.214.15";
    @GET("/api/post")
    io.reactivex.Observable<Post> getPost();
}

My model is this:

public class Post
{
    @SerializedName("id")
    private String id;
    @SerializedName("body")
    private String body;
    @SerializedName("title")
    private String title;

    public String getId ()
    {
        return id;
    }


    public String getBody ()
    {
        return body;
    }


    public String getTitle ()
    {
        return title;
    }

}

And this is what I did in my activity:

public class Javax extends AppCompatActivity {
    RecyclerView rvListContainer;
    postAdapter postAdapter;
    List<String> messageList=new ArrayList<>();
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_javax);

        rvListContainer=(RecyclerView)findViewById(R.id.recyclerView);
        postAdapter=new postAdapter(messageList);

        rvListContainer.setAdapter(postAdapter);
    }
    @Override
    protected void onResume() {
        super.onResume();
        Retrofit retrofit=new Retrofit.Builder()
                .addCallAdapterFactory(RxJavaCallAdapterFactory.create())
                .addConverterFactory(GsonConverterFactory.create())
                .baseUrl("https://198.50.214.15")
                .build();
        ApiCall apiService=retrofit.create(ApiCall.class);

        Observable<Post> observable=apiService.getPost();

        observable.subscribeOn(Schedulers.newThread())
                .observeOn(AndroidSchedulers.mainThread())
                .subscribe(responseData -> {
                    messageList.add(responseData.getTitle());
                    postAdapter.notifyDataSetChanged();
                });

    }
}

I don't know why I get this error which says I have problem with adapter. I also included the adapter into the gradle:

E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.hussein.sqlitedatabase, PID: 19445
java.lang.RuntimeException: Unable to resume activity {com.example.hussein.sqlitedatabase/com.example.hussein.sqlitedatabase.Javax}: java.lang.IllegalArgumentException: Unable to create call adapter for io.reactivex.Observable<com.example.hussein.sqlitedatabase.Post>
	for method ApiCall.getPost
	at android.app.ActivityThread.performResumeActivity(ActivityThread.java:2964)
	at android.app.ActivityThread.handleResumeActivity(ActivityThread.java:2993)
	at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2395)
	at android.app.ActivityThread.access$800(ActivityThread.java:151)
	at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1321)
	at android.os.Handler.dispatchMessage(Handler.java:110)
	at android.os.Looper.loop(Looper.java:193)
	at android.app.ActivityThread.main(ActivityThread.java:5292)
	at java.lang.reflect.Method.invokeNative(Native Method)
	at java.lang.reflect.Method.invoke(Method.java:515)
	at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:824)
	at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:640)
	at dalvik.system.NativeStart.main(Native Method)
 Caused by: java.lang.IllegalArgumentException: Unable to create call adapter for io.reactivex.Observable<com.example.hussein.sqlitedatabase.Post>
	for method ApiCall.getPost
	at retrofit2.ServiceMethod$Builder.methodError(ServiceMethod.java:751)
	at retrofit2.ServiceMethod$Builder.createCallAdapter(ServiceMethod.java:236)
	at retrofit2.ServiceMethod$Builder.build(ServiceMethod.java:161)
	at retrofit2.Retrofit.loadServiceMethod(Retrofit.java:169)
	at retrofit2.Retrofit$1.invoke(Retrofit.java:146)
	at $Proxy0.getPost(Native Method)
	at com.example.hussein.sqlitedatabase.Javax.onResume(Javax.java:42)
	at android.app.Instrumentation.callActivityOnResume(Instrumentation.java:1197)
	at android.app.Activity.performResume(Activity.java:5343)
	at android.app.ActivityThread.performResumeActivity(ActivityThread.java:2950)
		... 12 more
 Caused by: java.lang.IllegalArgumentException: Could not locate call adapter for io.reactivex.Observable<com.example.hussein.sqlitedatabase.Post>.
  Tried:
   * retrofit2.adapter.rxjava.RxJavaCallAdapterFactory
   * retrofit2.ExecutorCallAdapterFactory
	at retrofit2.Retrofit.nextCallAdapter(Retrofit.java:240)
	at retrofit2.Retrofit.callAdapter(Retrofit.java:204)
	at retrofit2.ServiceMethod$Builder.createCallAdapter(ServiceMethod.java:234)
		... 20 more

This is my Gradle dependency:

dependencies {
    compile fileTree(dir: 'libs', include: ['*.jar'])
    androidTestCompile('com.android.support.test.espresso:espresso-core:2.2.2', {
        exclude group: 'com.android.support', module: 'support-annotations'
    })
    compile 'com.android.support.constraint:constraint-layout:1.0.2'
    compile 'com.android.support:appcompat-v7:25.3.1'
    testCompile 'junit:junit:4.12'

    compile 'com.squareup.retrofit2:retrofit:2.2.0'

    compile 'com.android.support:recyclerview-v7:25.3.1'
    compile 'com.android.support:design:25.3.1'

    compile 'io.reactivex.rxjava2:rxandroid:2.0.1'
    compile 'io.reactivex.rxjava2:rxjava:2.0.1'

    compile 'com.jakewharton.retrofit:retrofit2-rxjava2-adapter:1.0.0'
    compile 'com.squareup.retrofit2:adapter-rxjava:2.0.2'

    compile 'com.squareup.retrofit2:converter-gson:2.0.0'

    compile 'com.google.code.gson:gson:2.4'

}

You need to tell Retrofit that you want to use RxJava 2, using:

addCallAdapterFactory(RxJava2CallAdapterFactory.create())

So, for creating your Retrofit object, you will have something like:

Retrofit retrofit = new Retrofit.Builder()
    .baseUrl(SERVICE_ENDPOINT)
    .addCallAdapterFactory(RxJava2CallAdapterFactory.create())
    .build();

I ran into the same issue. Instead of adding new repositories you can add the dependency of Jake Wharton's library:

implementation 'com.jakewharton.retrofit:retrofit2-rxjava2-adapter:1.0.0'

Then you can add factory:

Retrofit retrofit = new Retrofit.Builder()
    .baseUrl("https://api.example.com")
    .addCallAdapterFactory(RxJava2CallAdapterFactory.create())
    .build();

The fact that adapter has version 2.. does not mean that it is intended for use with RxJava 2, as I thought.

implementation 'com.squareup.retrofit2:adapter-rxjava2:2.1.0'

The problem is with the library being used.

I have replaced

implementation 'com.squareup.retrofit2:adapter-rxjava:2.4.0'

with

implementation 'com.squareup.retrofit2:adapter-rxjava2:2.4.0'

and used

Retrofit retrofit = new Retrofit.Builder()
                .baseUrl(BASE_URL).client(client)
                .addConverterFactory(GsonConverterFactory.create(gson))
                .addCallAdapterFactory(RxJava2CallAdapterFactory.create())
                .build();