What thread pool is used under the hood of Stream.parallel()? How does ForkJoinPool differ from FixedThreadPool?

Stream.parallel() in Java by default uses the common ForkJoinPool(). This thread pool is based on ForkJoinPool — a specialized pool for tasks that can be recursively broken down into subtasks.

Differences between ForkJoinPool and FixedThreadPool:

• ForkJoinPool is optimized for tasks that can be recursively split (fork) and then combined (join). It uses a work-stealing algorithm, where threads that finish their tasks can "steal" work from other threads, increasing efficiency.

• FixedThreadPool is a pool with a fixed number of threads that simply queues tasks and executes them sequentially without special optimizations for fork/join.

Example of creating a ForkJoinPool:

ForkJoinPool pool = new ForkJoinPool();
pool.submit(() -> {
    // parallel task
});

Thus, Stream.parallel() uses ForkJoinPool.commonPool(), which allows efficient parallelization of tasks using the fork/join mechanism.