RxJava fromCallable() and defer() difference in behaviour-CodePudding

I have implement a validate function, sample code are as below:

public class TestObject {
  LocalDateTime d1;
  LocalDateTime d2;

  //getters, setters
}

public class ErrorResult {
  String errorMsg;
  
  public ErrorResult(String errorMsg){
    this.errorMsg = errorMsg;
  }
}

public Observable<ErrorResult> validate(TestObject testObject){
  // implementation section
}

For the implementation section, I have tried to use Observable.fromCallable() and Observable.defer() functions to do the same job, for example:

// for implementation using Observable.fromCallable
return Observable.fromCallable(() -> {
  LocalDateTime d1 = testObject.getD1();
  LocalDateTime d2 = testObject.getD2();
  if(d1.isBefore(d2))
    return Observable.just(new ErrorResult("Error d1 is before d2"));

  return Observable.empty();
});

// for implementation using Observable.defer
return Observable.defer(() -> {
  LocalDateTime d1 = testObject.getD1();
  LocalDateTime d2 = testObject.getD2();
  if(d1.isBefore(d2))
    return Observable.just(new ErrorResult("Error d1 is before d2"));

  return Observable.empty();
});

While the implementation of the logic within the anonymous functions is the same,

the implementation using Observable.fromCallable throw a compilation error in IDE (Intellij), which stated "no instance(s) of type variable(s) T exist so that Observable conforms to ErrorResult". It was caused by the return of Observable.empty() as the fallback return in case no error exists
the implementation using Observable.defer compile successfully without error

May I know what is the difference between Observable.fromCallable() and Observable.defer() in this case?

Thanks a lot for the help

CodePudding user response：

fromCallable is not designed to return an Observable, while defer is. So in your example with fromCallable, you're ending up with a Observable<Observable<ErrorResult>>. You actually can't get your desired behavior with fromCallable, because you can't use it to get an empty Observable (returning null from the Callable results in a NullPointerException).