Provide Flux to signal events without a concrete value?

Solution 1:

Reactive streams only work with objects (you can never stream null), and since Void can never be instantiated you can therefore never send any values - just a completion signal.

I'm not quite sure of the use case here, but there's two different approaches you can take that spring to mind:

• If you never need the value, you can just map to an Object, i.e.

  Flux<Object> events = getFilesystemChanges().map(p -> new Object());
  

  You can then just use events.startWith(new Object()) and carry on. (You can pick a more appropriate type than Object if you want of course, or create your own.)

• If you need the actual values emitted, but sometimes need to just interject your own "dummy" values, then you can have a stream of Optional<Path> instead:

  Flux<Optional<Path>> events = getFilesystemChanges().map(Optional::of);
  

  Then you can do events.startWith(Optional.empty()) to prepend a "dummy" path to the stream.

FWIW, the second approach I've often used for "heartbeat" type functions, where I want a stream of values to be emitted, but also want to ensure there's activity once every (timeperiod) to keep an underlying connection alive. The latter is where I'll often use empty optionals.