Why is list.parallelStream().forEach() not processing all the elements in the list in Java?

Question

The following code is not putting all the elements in the destination list after finishing parallel process. Is there any reason for this?

public static void main(String[] args) {

    List<Integer> source =new ArrayList<>();
    List<Integer> destination = new ArrayList<>();
    IntStream.range(0, 10000).forEach(element ->{
        source.add(element);
    });

    //System.out.println(source.size());

    source.parallelStream().forEach(c->{
        destination.add(c);
    });

    System.out.println("Source Size:"+source.size());
    System.out.println("destination size:"+destination.size());
}

Output: Source Size:10000 destination size:4343

Answer 1

Because ArrayList is not a thread-safe collection. Using a thread-safe collection like CopyOnWriteArrayList would make it correct but not necessarily efficient.

Using a Collector instead would be much simpler and correct. e.g.

source.parallelStream().collect(Collectors.toList())

Answer 2

The forEach operation of the parallel stream is adding elements to an un-synchronized Collection (an ArrayList) from multiple threads. Therefore, the operation is not thread safe, and has unexpected results.

Using forEachOrdered() instead of forEach() will ensure all the elements of the source List are added to the destination List.

However, as mentioned in the other answer, using collect(Collectors.toList()) is the correct way to produce an output List from a Stream.