create an observable from callback.

createObservableFromCallback.java

public Observable<String> doAmazingThingObservable(int param) {
  return Observable.create(new Observable.OnSubscribe<String>() {

    @Override
    public void call(Subscriber<? super String> subscriber) {
      doAmazingThing(param, new CallBack() {

        @Override
        public void onGotResult(String result) {
          if (result == null) {
            if (!subscriber.isUnsubscribed()) {
              subscriber.onError(new SomeException());
            }
          } else {
            if (!subscriber.isUnsubscribed()) {
              subscriber.onNext(result);
              subscriber.onCompleted();
            }
          }
        }
      });
    }
  });
}