How to send elements to a Source.actorRef or a Source.queue in Java

I'm currently working with Akka Streams (in Java) for a personal project and I'm having a hard time understanding how to send element to a Source.

The idea is to use a WebSocket to push content into the user's web browser. I've managed to use Akka Streams to create a request-response system, following the Akka HTTP documentation, but this is not what I want to do.

Looking into the Akka Streams documentation, I saw that there is Source.queue and Source.actorRef. But I don't understand how to put an element into the Source. Source.queue and Source.actorRef return a Source, which doesn't have the method offer (for Source.queue) or tell (for Source.actorRef).

My question is: how do I get the ActorRef for the Source created by Source.actorRef or the SourceQueueWithComplete for a Source created with Source.queue, to be able to send elements to my Source?

I searched the various Akka documentation but found no method to do that. And the majority of the code I found on the Internet is written in Scala, which doesn't seem to have the same problem.

Solution

The actor and queue from Source.actorRef and Source.queue, respectively, are the materialized values of those sources, meaning that they can be obtained only if the stream is running. For example:

final ActorRef actor =
  Source.actorRef(Integer.MAX_VALUE, OverflowStrategy.fail())
        .to(Sink.foreach(m -> System.out.println(m)))
        .run(materializer);

actor.tell("do something", ActorRef.noSender());

It's no different in Scala:

implicit val materializer = ActorMaterializer()

val actor =
  Source.actorRef(Int.MaxValue, OverflowStrategy.fail)
        .to(Sink.foreach(println))
        .run()

actor ! "do something"