The entry object of the first element in the queue.
Each element of the queue has an associated DoubleLinkedQueueEntry. Returns the entry object corresponding to the first element of the queue.
The entry objects can also be accessed using lastEntry,
and they can be iterated using DoubleLinkedQueueEntry.nextEntry() and
DoubleLinkedQueueEntry.previousEntry().
Source
DoubleLinkedQueueEntry<E> firstEntry() {
return _sentinel.nextEntry();
}