title: java.lang.NoClassDefFoundError: io.reactivex.Observable
body: <p>When I use Observable in my retrofit API on the phone with API 21 I got this error:</p>
  
  <pre><code>java.lang.NoClassDefFoundError: io.reactivex.Observable
  </code></pre>
  
  <p>but on the phone with API 19 or emulator with API 23 it works.</p>
  
  <p>This is my API Interface:</p>
  
  <pre><code>import io.reactivex.Observable;
  import retrofit2.Response;
  import retrofit2.http.Body;
  import retrofit2.http.POST;
  public interface ApiService {
      /**
       * Created by Mohsen on 5/10/2017.
       *
       */
      @POST("/cp/api/")
      Observable&lt;Response&lt;Integer&gt;&gt; Get_BuyBox_Count(@Body Object request);
  }
  </code></pre>
  
  <p>and this is my Retrofit setting:</p>
  
  <pre><code>    @Provides
      @Application_Scope
      @Store_Retrofit_Qualifier
      public Retrofit Store_retrofit(OkHttpClient client) {
          return new Retrofit.Builder()
                  .baseUrl(Urls.Sotre_Base_Url)
                  .client(client)
                  .addCallAdapterFactory(RxJava2CallAdapterFactory.create())
                  .addConverterFactory(JSONConverterFactory.create())
                  .addConverterFactory(GsonConverterFactory.create())
                  .build();
      }
  </code></pre>
  
  <p>my Dependencies:</p>
  
  <pre><code>    compile 'com.squareup.retrofit2:retrofit:2.2.0'
      compile 'com.squareup.retrofit2:converter-gson:2.1.0'
      compile 'io.reactivex.rxjava2:rxjava:2.0.1'
      compile 'io.reactivex.rxjava2:rxandroid:2.0.1'
      compile 'com.jakewharton.retrofit:retrofit2-rxjava2-adapter:1.0.0'
  </code></pre>