Thread in stream vs Parallel Stream

Question

I am unsure about which to use in performance wise. Which should be used? Both version have the some functionality.

Code snippet 1

fileMap.values().stream().parallel().forEach(file -> {
    downlod(file);
});

Code snippet 2

fileMap.values().forEach(file -> {
    executor.execute(file);
}

Answer 1

In case of heavy computation task, go with separate executor. parallelStream uses shared thread pool, thus heavy tasks can potentially block parallelStreams in other parts of application.