public E poll() {
restartFromHead:
for (; ; ) {
for (Node<E> h = head, p = h, q; ; ) {
E item = p.item;
if (item != null && p.casItem(item, null)) {
// Successful CAS is the linearization point
// for item to be removed from this queue.
if (p != h) // hop two nodes at a time
updateHead(h, ((q = p.next) != null) ? q : p);
return item;
} else if ((q = p.next) == null) {
updateHead(h, p);
return null;
} else if (p == q) continue restartFromHead;
else p = q;
}
}
}
/**
* Returns the first live (non-deleted) node on list, or null if none. This is yet another variant
* of poll/peek; here returning the first node, not element. We could make peek() a wrapper around
* first(), but that would cost an extra volatile read of item, and the need to add a retry loop
* to deal with the possibility of losing a race to a concurrent poll().
*/
Node<E> first() {
restartFromHead:
for (; ; ) {
for (Node<E> h = head, p = h, q; ; ) {
boolean hasItem = (p.item != null);
if (hasItem || (q = p.next) == null) {
updateHead(h, p);
return hasItem ? p : null;
} else if (p == q) continue restartFromHead;
else p = q;
}
}
}
public E peek() {
restartFromHead:
for (; ; ) {
for (Node<E> h = head, p = h, q; ; ) {
E item = p.item;
if (item != null || (q = p.next) == null) {
updateHead(h, p);
return item;
} else if (p == q) continue restartFromHead;
else p = q;
}
}
}