queue.go
func (q *Queue) Length() uint {
	q.key.RLock()
	defer q.key.RUnlock()
	if nil == q.underlyer {
		return 0
	}
	return q.underlyer.length
}
func (q *Queue) Length() uint {
	q.key.RLock()
	defer q.key.RUnlock()
	if nil == q.underlyer {
		return 0
	}
	return q.underlyer.length
}
// Length returns the number of items in the Queue.