Queue Implementations
The Queue
implementations are grouped into general-purpose and concurrent implementations.
General-Purpose Queue Implementations
As mentioned in the previous section, LinkedList
implements the Queue
interface, providing first in, first out (FIFO) queue operations for add
, poll
, and so on.
The PriorityQueue
class is a priority queue based on the heap data structure. This queue orders elements according to the order specified at construction time, which can be the elements' natural ordering or the ordering imposed by an explicit Comparator
.
The queue retrieval operations — poll
, remove
, peek
, and element
— access the element at the head of the queue. The head of the queue is the least element with respect to the specified ordering. If multiple elements are tied for least value, the head is one of those elements; ties are broken arbitrarily.
PriorityQueue
and its iterator implement all of the optional methods of the Collection
and Iterator
interfaces. The iterator provided in method iterator
is not guaranteed to traverse the elements of the PriorityQueue
in any particular order. For ordered traversal, consider using Arrays.sort(pq.toArray())
.
Concurrent Queue Implementations
The java.util.concurrent
package contains a set of synchronized Queue
interfaces and classes. BlockingQueue
extends Queue
with operations that wait for the queue to become nonempty when retrieving an element and for space to become available in the queue when storing an element. This interface is implemented by the following classes:
-
LinkedBlockingQueue
— an optionally bounded FIFO blocking queue backed by linked nodes -
ArrayBlockingQueue
— a bounded FIFO blocking queue backed by an array -
PriorityBlockingQueue
— an unbounded blocking priority queue backed by a heap -
DelayQueue
— a time-based scheduling queue backed by a heap -
SynchronousQueue
— a simple rendezvous mechanism that uses theBlockingQueue
interface
In JDK 7, TransferQueue
is a specialized BlockingQueue
in which code that adds an element to the queue has the option of waiting (blocking) for code in another thread to retrieve the element. TransferQueue
has a single implementation:
-
LinkedTransferQueue
— an unboundedTransferQueue
based on linked nodes