Why this BiConsumer Combiner code in collect method is never reached?

Question

In the main method,I have made a list of Person with different ages.Now when I use collect method to transform this list into a list of Persons' ages.The code in BiConsumer combiner function is never reached.

class Person {
    private int age;

    public int getAge() {
        return age;
    }
}

//second flavor of collect

ArrayList<Integer> pAges = people.stream()
                            .collect(ArrayList<Integer>::new, 
                                    (listGivenBySupplier, personObjectFromPeopleStream) -> listGivenBySupplier.add(personObjectFromPeopleStream.getAge()), 
                                    (r1, r2) -> {  //Also please explain what value is passed to r1 and r2
                                            System.out.println("r1: " + r1);
                                            System.out.println("r2: " + r2);
                                            r1.add(2222);
                                            r2.add(2211); 
                            });
System.out.println("pAges:" + pAges);

you could just use stream().map() as next: List ages = people.stream().map(Person::getAge).collect(Collectors.toList()); — lub0v, Jul 31 '19 at 05:02
@lub0v I know that, but I want to use the other flavor of collect() with supplier, biconsumer accumulator and biconsumer combiner. — Codeninja, Jul 31 '19 at 05:21
here is the same question, btw https://stackoverflow.com/questions/29959795/how-does-combiner-in-stream-collect-method-work-in-java-8 — lub0v, Jul 31 '19 at 05:27

Ravindra Ranwala · Accepted Answer · 2019-07-31T06:06:27.843

1

The combiner function is called only if you invoke the stream processing pipeline in parallel. So change it to a parallel stream and then it should reach the combiner function. So this should trigger it.

people.parallelStream()...

For an instance assume there are 2 worker threads handling this workload. Any parallel execution involves spliting the source into parts, executing them concurently and finally merging the partial results into one single result container. Each thread T1 and T2 has an associated List, such that the container is thread confined. The accumulator function adds each single element into the associated container. Upon completion of both threads T1 and T2, the partial containers should be merged into one large result container. That's where the combiner function comes into play. In a serial execution such a merging of result is not involved, hence combiner has no use there.

edited Jul 31 '19 at 06:06

answered Jul 31 '19 at 05:22

Ravindra Ranwala

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thanks man it's working now, but could you explain why this works only with parallel streams and not sequential ? – Codeninja Jul 31 '19 at 05:24
2

Parallel, means using _splitting_ in parts. Afterwards the entire result must be _combined_, say several partial lists into an entire one. – Joop Eggen Jul 31 '19 at 05:27
@JoopEggen ok now I get it , thanks – Codeninja Jul 31 '19 at 05:28

Why this BiConsumer Combiner code in collect method is never reached?

1 Answers1