Generate one Stream from multiple Suppliers

Question

I'm trying to use Java 8 Lambda/Stream API for modeling a simple producer/consumer system, just like:

    Stream<Sample> samplesFlow = Stream.generate(new SampleSupplier());
    samplesFlow.forEach(new SampleConsumer());

I realized that "scale" to several consumers was pretty simple:

    samplesFlow
        .peek(new SampleConsumer1())
        .peek(new SampleConsumer2())
        .forEach(new SampleConsumer3());

But what about adding new producers to the system? Is there some idiomatic or "elegant" way to generate a Stream from several infinite Suppliers?? Like:

Stream.generate(new SampleSupplier1(),new SampleSupplier2());  // made it up

Each supplier simulate a network listener getting data from a remote source.

Thanks in advance!

Answer 1

You didn’t specify how you want to combine the supplied values.

If you want to have the values alternating, a solution would be:

Stream.generate(supplier1).flatMap(x->Stream.of(x, supplier2.get()))

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If you want to have some kind of pair, you can use

Stream.generate(()->new Pair<>(supplier1.get(), supplier2.get()))

though it is up to you to write the Pair class as the jdk does not offer such a class. (You could abuse AbstractMap.SimpleEntry but that’s rather nasty).

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If you have a finite Stream you can use Stream.concat to create a stream which will process all items of the first stream before the items of the second, however, streams created using a Supplier are infinite by default so you have to use limit on the first stream before you can concat it to another stream, so it’s not a general solution.

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If you want to query each Supplier exactly once, you can use

Stream.of(supplier1, supplier2).map(Supplier::get)

though, of course, if you don’t need lazy evaluation of the Suppliers, a

Stream.of(supplier1.get(), supplier2.get())

would do as well.