Java 8 Streams - Sequential vs Parallel streams - (Parallel thru Fork & Join)

Parallel streams divide the provided task into many and run them in different threads, utilizing multiple cores of the computer. On the other hand sequential streams work just like for-loop using a single core.

The tasks provided to the streams are typically the iterative operations performed on the elements of a collection or array or from other dynamic sources. Parallel execution of streams run multiple iterations simultaniously in different available core

In parallel execution, if number of tasks are more than available cores at a given time, the remaining tasks are queued waiting for currently running task to finish.

It is also important to know that iterations are only performed at a terminal operation, that's because streams are deisnged to be lazy.

Below is the program that in which clearly shows that in sequential stream each iteration waits for currently running one to finish, whereas, in parallel stream, eight threads are spawn simultaneously, remaining two, wait for others. Also notice the name of threads. In parallel stream, Fork and Join framework is used to create multiple threads. Parallel streams create ForkJoinPool instance via static ForkJoinPool.commonPool() method.