Non-preemptive multitasking in Java (Coroutine, Fibers), why not Callable?

Question

Java

I watched the video www.youtube.com/watch?v=YWLHELcvNbE
the question is. Why not just use Callable?

DbA.get(id, new OnCompleted(){ 
                      @override 
                       success(MyResult result){ ..; }
                       error(NotFoundException e){ ..; }
                       error(Exception e){ ..; }
                 }
         );

Answer 1

callback hell. We do not want to rewrite the code especially because of the execution model.