Replace callbacks with observables from RxJava

For example you can use Observable.fromCallable to create observable with your data.

public Observable<Data> getData(){
    return Observable.fromCallable(() -> {
        Data result = null;
        //do something, get your Data object
        return result;
    });
}

then use your data

 getData().subscribeOn(Schedulers.io())
            .observeOn(AndroidSchedulers.mainThread())
            .subscribe(data -> {
                //do something with your data
            }, error -> {
                //do something on error
            });

Used rxjava 1.x and lambda expressions.

edit:

if I understand you well, you wanted to replace that listener, not wrap it into observable. I added other example in reference to your comment. Oh.. also you should use Single if you are expecting only one item.

public Single<Data> getData() {
        return Single.create(singleSubscriber -> {
            Data result = object.getData();
            if(result == null){
                singleSubscriber.onError(new Exception("no data"));
            } else {
                singleSubscriber.onSuccess(result);
            }
        });
    }

getData().subscribeOn(Schedulers.io())
            .observeOn(AndroidSchedulers.mainThread())
            .subscribe(data -> {
                //do something with your data
            }, error -> {
                //do something on error
            });