Are lambda-expressions in Java8 executed multi threaded?

Question

In Java8, lambda expressions were introduced. This question is about when parallel lambdas are executed.

Before Java8, Callable-classes were one way to execute multiple threads at one time. Callables can be used with Executor-classes to be executed. Let's assume I am using a Fixed Thread Pool, using 3 as number of active processing tasks. And let's assume I have 8 tasks. Fixed Thread Pool would start first three tasks, and as one finishes, next task is started, until all 8 tasks are finished.

What would happen, if I implement my tasks as Java8-lambdas? Would all 8 be started at once? Or sequentially? Or in any clever way?

In special, are they running in the same thread as the caller (without using an Exeuctor)? By their nature, I guess lambdas could be executed in another thread easily.

Answer 1

Runnable r = () -> System.out.println("hello");

is equivalent to

Runnable r = new Runnable() {
    @Override
    public void run() {
        System.out.println("hello")
    }
};

And it doesn't change anything to how a runnable is executed. If you submit your runnable to a thread pool, then the thread pool will execute it, whatever you used to create your runnable: a lambda, an anonymous class, or a top-level class. In the end, what you define is an instance of Runnable, and that's the only thing which matters.