code: public Observable<ArrayList<SoundCloudComment>> getTrackComments(final SoundCloudTrack track) {
          checkState();
          if (mCacheRam.tracksComments.get(track.getId()) != null) {
              return Observable.create(new Observable.OnSubscribe<ArrayList<SoundCloudComment>>() {
                  @Override
                  public void call(Subscriber<? super ArrayList<SoundCloudComment>> subscriber) {
                      subscriber.onNext(mCacheRam.tracksComments.get(track.getId()));
                      subscriber.onCompleted();
                  }
              });
          } else {
              return mRetrofitService.getTrackComments(track.getId())
                      .map(RxParser.PARSE_COMMENTS)
                      .map(cacheTrackComments());
          }
      }
docs: Retrieve comments related to a track of the supported artist.
  
  @param track track of which comment are related.
  @return {@link rx.Observable} on {@link java.util.ArrayList}
  of {@link SoundCloudComment}