Java stream with parallel elements and sequential elements

Question

I want to run a Java stream in which the first element filters in parallel to the second and third element. The second and third element can not filter parallel, because the use a shared ressource.

int numberOfFilteredElements = Stream.of(firstElement, secondElememt, thirdElement)
  .filter(element -> element.someFilter())
  .collect(Collectors.toSet())
  .size();

A Java stream can be parallel or sequential. Concating streams are only parallel if all streams are parallel. So I can not create create a parallel and sequential stream and concat them.

Any other idea how I can solve this with streams? Or do I need to need use locks like ReentrantLock in someFilter() and run the stream parallel for all elements?

Answer 1

We can say:

• First element is Stream #1 .
• Second and third elements are Stream #2 .

Then we can create a Parent Parallel Stream that consists of Stream #1 , Stream #2 :

    Predicate<Integer> someFilter = i -> {
        try { Thread.sleep(4000); } catch (InterruptedException e) {e.printStackTrace();}
        logger.info(i + " filtered on thread id: " + Thread.currentThread().getId());
        return i != 7;
    };
    Stream<Integer> stream1 = IntStream.rangeClosed(0, 3).boxed().filter(someFilter);
    Stream<Integer> stream2 = IntStream.rangeClosed(-3, -1).boxed().filter(someFilter);

    int numberOfFilteredElements = Stream.of(stream1, stream2)
            .parallel()
            .flatMap(Function.identity())
            .peek(integer -> logger.info("Peeked: " + integer.toString()))
            .collect(Collectors.toSet())
            .size();
    System.out.println("Set size is: " + numberOfFilteredElements);

Output:

2021-04-01 12:56:34.084  INFO 26336 --- [           main] com.example.demo.DemoApplication         : Started DemoApplication in 2.966 seconds (JVM running for 5.717)
2021-04-01 12:56:38.098  INFO 26336 --- [           main] com.example.demo.DemoApplication         : -3 filtered on thread id: 1
2021-04-01 12:56:38.098  INFO 26336 --- [onPool-worker-1] com.example.demo.DemoApplication         : 0 filtered on thread id: 22
2021-04-01 12:56:38.100  INFO 26336 --- [           main] com.example.demo.DemoApplication         : Peeked: -3
2021-04-01 12:56:38.100  INFO 26336 --- [onPool-worker-1] com.example.demo.DemoApplication         : Peeked: 0
2021-04-01 12:56:42.100  INFO 26336 --- [onPool-worker-1] com.example.demo.DemoApplication         : 1 filtered on thread id: 22
2021-04-01 12:56:42.100  INFO 26336 --- [           main] com.example.demo.DemoApplication         : -2 filtered on thread id: 1
2021-04-01 12:56:42.100  INFO 26336 --- [           main] com.example.demo.DemoApplication         : Peeked: -2
2021-04-01 12:56:42.100  INFO 26336 --- [onPool-worker-1] com.example.demo.DemoApplication         : Peeked: 1
2021-04-01 12:56:46.101  INFO 26336 --- [onPool-worker-1] com.example.demo.DemoApplication         : 2 filtered on thread id: 22
2021-04-01 12:56:46.101  INFO 26336 --- [           main] com.example.demo.DemoApplication         : -1 filtered on thread id: 1
2021-04-01 12:56:46.101  INFO 26336 --- [           main] com.example.demo.DemoApplication         : Peeked: -1
2021-04-01 12:56:46.101  INFO 26336 --- [onPool-worker-1] com.example.demo.DemoApplication         : Peeked: 2
2021-04-01 12:56:50.102  INFO 26336 --- [onPool-worker-1] com.example.demo.DemoApplication         : 3 filtered on thread id: 22
2021-04-01 12:56:50.102  INFO 26336 --- [onPool-worker-1] com.example.demo.DemoApplication         : Peeked: 3
Set size is: 7

Notice that Stream #1 (which can be your first element) runs in parallel to Stream #2 (which can be second and third elements).