collecting from parallel stream in java 8

Question

I want to take an input and apply parallel stream on that, then I want output as list. Input could be any List or any collection on which we can apply streams.

My co

Answer 1

But there is no option that I can see to collect from parallel stream in thread safe way to provide list as output. This is entirely wrong.

The whole point in streams is that you can use a non-thread safe Collection to achieve perfectly valid thread-safe results. This is because of how streams are implemented (and this was a key part of the design of streams). You could see that a Collector defines a method supplier that at each step will create a new instance. Those instances will be merged between them.

So this is perfectly thread safe:

 Stream.of(1,2,3,4).parallel()
          .collect(Collectors.toList());

Since there are 4 elements in this stream, there will be 4 instances of ArrayList created that will be merged at the end to a single result (assuming at least 4 CPU cores)

On the other side methods like toConcurrent generate a single result container and all threads will put their result into it.

Answer 2

The Collection object used to receive the data being collected does not need to be concurrent. You can give it a simple ArrayList.

That is because the collection of values from a parallel stream is not actually collected into a single Collection object. Each thread will collect their own data, and then all sub-results will be merged into a single final Collection object.

This is all well-documented in the Collector javadoc, and the Collector is the parameter you're giving to the collect() method:

<R,A> R collect(Collector<? super T,A,R> collector)