Waits if necessary for at most the given time for the computation to complete, and then retrieves its result, if available.

Parameters:

timeout		the maximum time to wait
unit		the time unit of the timeout argument

Returns:  the computed result

Exceptions:

CancellationException		if the computation was cancelled
ExecutionException		if the computation threw an exception
InterruptedException		if the current thread was interrupted while waiting
TimeoutException		if the wait timed out