Can invokeAll be used within a RecursiveTask?

Question

I have read that the following code:

invokeAll(a2, a1);

is equivalent to:

a2.compute();
a1.join();

in a RecursiveAction in a Fork-Join.

However, since RecursiveTask returns a value we may have something like:

Integer result1 = t2.compute();
Integer result2 = t1.join();

And then we should merge result1 and result2 to produce the final result.

Now, my doubts are:

• Can invokedAll be used for RecursiveTasks?

• If so, how can the result of invokeAll calls be merged to return the final result?

Answer 1

The invokeAll method can be used for RecursiveTasks. It takes two ForkJoinTasks as parameters, and because RecursiveTask extends ForkJoinTask, a RecursiveTask can be used as a ForkJoinTask.

As for getting the results, invokeAll doesn't return anything, so you're not going to get the information that way. What you can do is call the join method on each of your RecursiveTasks. Because you know that both tasks have completed when invokeAll returns, the join method will return very quickly.

invokeAll(t1, t2);
Integer result1 = t1.join();
Integer result2 = t2.join();

However, at this point, you may as well just do the thing you were doing originally, calling compute and join. Using invokeAll is a bit clumsy when you only have two tasks and you need both of their results.