java.util.concurrent
public class: ConcurrentLinkedQueue [javadoc |
source]
java.lang.Object
java.util.AbstractCollection
java.util.AbstractQueue
java.util.concurrent.ConcurrentLinkedQueue
All Implemented Interfaces:
Queue, Serializable, Collection
An unbounded thread-safe {@linkplain Queue queue} based on linked nodes.
This queue orders elements FIFO (first-in-first-out).
The
head of the queue is that element that has been on the
queue the longest time.
The
tail of the queue is that element that has been on the
queue the shortest time. New elements
are inserted at the tail of the queue, and the queue retrieval
operations obtain elements at the head of the queue.
A
ConcurrentLinkedQueue is an appropriate choice when
many threads will share access to a common collection.
This queue does not permit
null elements.
This implementation employs an efficient "wait-free"
algorithm based on one described in Simple,
Fast, and Practical Non-Blocking and Blocking Concurrent Queue
Algorithms by Maged M. Michael and Michael L. Scott.
Beware that, unlike in most collections, the size method
is NOT a constant-time operation. Because of the
asynchronous nature of these queues, determining the current number
of elements requires a traversal of the elements.
This class and its iterator implement all of the
optional methods of the Collection and Iterator interfaces.
Memory consistency effects: As with other concurrent
collections, actions in a thread prior to placing an object into a
{@code ConcurrentLinkedQueue}
happen-before
actions subsequent to the access or removal of that element from
the {@code ConcurrentLinkedQueue} in another thread.
This class is a member of the
Java Collections Framework.
Parameters:
< - E> the type of elements held in this collection
- since:
1.5 -
- author:
Doug - Lea
| Constructor: |
|---|
 public ConcurrentLinkedQueue() {
}
Creates a ConcurrentLinkedQueue that is initially empty. |
public ConcurrentLinkedQueue(Collection c) {
for (Iterator< ? extends E > it = c.iterator(); it.hasNext();)
add(it.next());
} Creates a ConcurrentLinkedQueue
initially containing the elements of the given collection,
added in traversal order of the collection's iterator. Parameters:
c - the collection of elements to initially contain
NullPointerException - if the specified collection or any
of its elements are null
|
| Method from java.util.concurrent.ConcurrentLinkedQueue Summary: |
|---|
|
add, contains, first, isEmpty, iterator, offer, peek, poll, remove, size, toArray, toArray |
| Methods from java.util.AbstractCollection: |
|---|
|
add, addAll, clear, contains, containsAll, isEmpty, iterator, remove, removeAll, retainAll, size, toArray, toArray, toString |
| Methods from java.lang.Object: |
|---|
|
clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait |
| Method from java.util.concurrent.ConcurrentLinkedQueue Detail: |
|---|
public boolean add(E e) {
return offer(e);
} Inserts the specified element at the tail of this queue. |
public boolean contains(Object o) {
if (o == null) return false;
for (Node< E > p = first(); p != null; p = p.getNext()) {
E item = p.getItem();
if (item != null &&
o.equals(item))
return true;
}
return false;
} Returns true if this queue contains the specified element.
More formally, returns true if and only if this queue contains
at least one element e such that o.equals(e). |
ConcurrentLinkedQueue.Node first() {
for (;;) {
Node< E > h = head;
Node< E > t = tail;
Node< E > first = h.getNext();
if (h == head) {
if (h == t) {
if (first == null)
return null;
else
casTail(t, first);
} else {
if (first.getItem() != null)
return first;
else // remove deleted node and continue
casHead(h, first);
}
}
}
} Returns the first actual (non-header) node on list. This is yet
another variant of poll/peek; here returning out the first
node, not element (so we cannot collapse with peek() without
introducing race.) |
public boolean isEmpty() {
return first() == null;
} Returns true if this queue contains no elements. |
public Iterator iterator() {
return new Itr();
} Returns an iterator over the elements in this queue in proper sequence.
The returned iterator is a "weakly consistent" iterator that
will never throw ConcurrentModificationException ,
and guarantees to traverse elements as they existed upon
construction of the iterator, and may (but is not guaranteed to)
reflect any modifications subsequent to construction. |
public boolean offer(E e) {
if (e == null) throw new NullPointerException();
Node< E > n = new Node< E >(e, null);
for (;;) {
Node< E > t = tail;
Node< E > s = t.getNext();
if (t == tail) {
if (s == null) {
if (t.casNext(s, n)) {
casTail(t, n);
return true;
}
} else {
casTail(t, s);
}
}
}
} Inserts the specified element at the tail of this queue. |
public E peek() {
// same as poll except don't remove item
for (;;) {
Node< E > h = head;
Node< E > t = tail;
Node< E > first = h.getNext();
if (h == head) {
if (h == t) {
if (first == null)
return null;
else
casTail(t, first);
} else {
E item = first.getItem();
if (item != null)
return item;
else // remove deleted node and continue
casHead(h, first);
}
}
}
} |
public E poll() {
for (;;) {
Node< E > h = head;
Node< E > t = tail;
Node< E > first = h.getNext();
if (h == head) {
if (h == t) {
if (first == null)
return null;
else
casTail(t, first);
} else if (casHead(h, first)) {
E item = first.getItem();
if (item != null) {
first.setItem(null);
return item;
}
// else skip over deleted item, continue loop,
}
}
}
} |
public boolean remove(Object o) {
if (o == null) return false;
for (Node< E > p = first(); p != null; p = p.getNext()) {
E item = p.getItem();
if (item != null &&
o.equals(item) &&
p.casItem(item, null))
return true;
}
return false;
} Removes a single instance of the specified element from this queue,
if it is present. More formally, removes an element e such
that o.equals(e), if this queue contains one or more such
elements.
Returns true if this queue contained the specified element
(or equivalently, if this queue changed as a result of the call). |
public int size() {
int count = 0;
for (Node< E > p = first(); p != null; p = p.getNext()) {
if (p.getItem() != null) {
// Collections.size() spec says to max out
if (++count == Integer.MAX_VALUE)
break;
}
}
return count;
} Returns the number of elements in this queue. If this queue
contains more than Integer.MAX_VALUE elements, returns
Integer.MAX_VALUE.
Beware that, unlike in most collections, this method is
NOT a constant-time operation. Because of the
asynchronous nature of these queues, determining the current
number of elements requires an O(n) traversal. |
public Object[] toArray() {
// Use ArrayList to deal with resizing.
ArrayList< E > al = new ArrayList< E >();
for (Node< E > p = first(); p != null; p = p.getNext()) {
E item = p.getItem();
if (item != null)
al.add(item);
}
return al.toArray();
} Returns an array containing all of the elements in this queue, in
proper sequence.
The returned array will be "safe" in that no references to it are
maintained by this queue. (In other words, this method must allocate
a new array). The caller is thus free to modify the returned array.
This method acts as bridge between array-based and collection-based
APIs. |
public T[] toArray(T[] a) {
// try to use sent-in array
int k = 0;
Node< E > p;
for (p = first(); p != null && k < a.length; p = p.getNext()) {
E item = p.getItem();
if (item != null)
a[k++] = (T)item;
}
if (p == null) {
if (k < a.length)
a[k] = null;
return a;
}
// If won't fit, use ArrayList version
ArrayList< E > al = new ArrayList< E >();
for (Node< E > q = first(); q != null; q = q.getNext()) {
E item = q.getItem();
if (item != null)
al.add(item);
}
return al.toArray(a);
} Returns an array containing all of the elements in this queue, in
proper sequence; the runtime type of the returned array is that of
the specified array. If the queue fits in the specified array, it
is returned therein. Otherwise, a new array is allocated with the
runtime type of the specified array and the size of this queue.
If this queue fits in the specified array with room to spare
(i.e., the array has more elements than this queue), the element in
the array immediately following the end of the queue is set to
null.
Like the #toArray() method, this method acts as bridge between
array-based and collection-based APIs. Further, this method allows
precise control over the runtime type of the output array, and may,
under certain circumstances, be used to save allocation costs.
Suppose x is a queue known to contain only strings.
The following code can be used to dump the queue into a newly
allocated array of String:
String[] y = x.toArray(new String[0]);
Note that toArray(new Object[0]) is identical in function to
toArray(). |
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