What is the difference if I use just instead of fromCallable?

Observable.fromCallable(new Callable<_User>() {
    @Override
    public _User call() throws Exception {
        return UsersService.getUser(123);
    }
})
      .subscribeOn(Schedulers.io())
      .observeOn(AndroidSchedulers.mainThread())
      .subscribe(new Subscriber<_User>() {
          @Override
          public void onCompleted() {
              Log.d("Rx", "Callable Completed");
          }

          @Override
          public void onError(Throwable e) {
              Log.d("Rx", e.getMessage());
          }

          @Override
          public void onNext(_User user) {
              Log.d("Rx", user.getName());
          }
      });

Observable.just(UsersService.getUser(123))
      .subscribeOn(Schedulers.io())
      .observeOn(AndroidSchedulers.mainThread())
      .subscribe(new Subscriber<_User>() {
          @Override
          public void onCompleted() {
              Log.d("Rx", "Just Completed");
          }

          @Override
          public void onError(Throwable e) {
              Log.d("Rx", e.getMessage());
          }

          @Override
          public void onNext(_User user) {
              Log.d("Rx", user.getName());
          }
      });