Empty

create an Observable that emits no items but terminates normally

Empty

Never

create an Observable that emits no items and does not terminate

Never

Throw

create an Observable that emits no items and terminates with an error

Throw

The Empty, Never, and Throw operators generate Observables with very specific and limited behavior. These are useful for testing purposes, and sometimes also for combining with other Observables or as parameters to operators that expect other Observables as parameters.

See Also

Language-Specific Information:

RxGroovy implements these operators as empty, never, and error. The error operator takes as a parameter the Throwable with which you want the Observable to terminate.

These operators do not operate by default on any particular Scheduler, but empty and error optionally take a Scheduler as a parameter, and if you pass them a Scheduler they will issue their termination notifications on that Scheduler.

Sample Code

println("*** empty() ***");
Observable.empty().subscribe(
  { println("empty: " + it); },                      // onNext
  { println("empty: error - " + it.getMessage()); }, // onError
  { println("empty: Sequence complete"); }           // onCompleted
);

println("*** error() ***");
Observable.error(new Throwable("badness")).subscribe(
  { println("error: " + it); },                      // onNext
  { println("error: error - " + it.getMessage()); }, // onError
  { println("error: Sequence complete"); }           // onCompleted
);

println("*** never() ***");
Observable.never().subscribe(
  { println("never: " + it); },                      // onNext
  { println("never: error - " + it.getMessage()); }, // onError
  { println("never: Sequence complete"); }           // onCompleted
);
println("*** END ***");
*** empty() ***
empty: Sequence complete
*** error() ***
error: error - badness
*** never() ***
*** END ***

RxJava 1.x implements these operators as empty, never, and error. The error operator takes as a parameter the Throwable with which you want the Observable to terminate.

These operators do not operate by default on any particular Scheduler, but empty and error optionally take a Scheduler as a parameter, and if you pass them a Scheduler they will issue their termination notifications on that Scheduler.

RxJava 2.x implements these operators as empty, never, and error. The error operator takes as a parameter the Throwable with which you want the Observable to terminate, or a Callable that returns such a Throwable.

These operators do not operate by default on any particular Scheduler.

RxJS implements these operators as empty, never, and throw.

Sample Code

var source = Rx.Observable.empty();

var subscription = source.subscribe(
    function (x) { console.log('Next: ' + x); },
    function (err) { console.log('Error: ' + err); },
    function () { console.log('Completed'); });
Completed
// This will never produce a value, hence never calling any of the callbacks
var source = Rx.Observable.never();

var subscription = source.subscribe(
    function (x) { console.log('Next: ' + x); },
    function (err) { console.log('Error: ' + err); },
    function () { console.log('Completed'); });
var source = Rx.Observable.return(42)
    .selectMany(Rx.Observable.throw(new Error('error!')));

var subscription = source.subscribe(
    function (x) { console.log('Next: ' + x); },
    function (err) { console.log('Error: ' + err); },
    function () { console.log('Completed'); });
Error: Error: error!

empty is found in the following distributions:

  • rx.js
  • rx.all.js
  • rx.all.compat.js
  • rx.compat.js
  • rx.lite.js
  • rx.lite.compat.js

never is found in the following distributions:

  • rx.js
  • rx.compat.js
  • rx.lite.js
  • rx.lite.compat.js

throw is found in the following distributions:

  • rx.js
  • rx.all.js
  • rx.compat.js
  • rx.lite.js
  • rx.lite.compat.js

RxPHP implements this operator as emptyObservable.

Returns an empty observable sequence.

Sample Code

//from https://github.com/ReactiveX/RxPHP/blob/master/demo/empty-observable/empty-observable.php

$observable = \Rx\Observable::emptyObservable();
$observable->subscribe($stdoutObserver);

   
Complete!
    

RxPHP also has an operator never.

Returns a non-terminating observable sequence, which can be used to denote an infinite duration.

Sample Code

//from https://github.com/ReactiveX/RxPHP/blob/master/demo/never/never.php

$observable = \Rx\Observable::never();
$observable->subscribe($stdoutObserver);

   

    

RxPHP also has an operator error.

Returns an observable sequence that terminates with an exception.

Sample Code

//from https://github.com/ReactiveX/RxPHP/blob/master/demo/error-observable/error-observable.php

$observable = Rx\Observable::error(new Exception('Oops!'));
$observable->subscribe($stdoutObserver);

   
Exception: Oops!