Returns the result value (or throws any encountered exception)
if completed, else returns the given valueIfAbsent.
@param valueIfAbsent the value to return if not completed
@return the result value, if completed, else the given valueIfAbsent
@throws CancellationException if the computation was cancelled
@throws CompletionException if this future completed
exceptionally or a completion computation threw an exception
Returns the result value (or throws any encountered exception) if completed, else returns the given valueIfAbsent.
@param valueIfAbsent the value to return if not completed @return the result value, if completed, else the given valueIfAbsent @throws CancellationException if the computation was cancelled @throws CompletionException if this future completed exceptionally or a completion computation threw an exception