Queues in Java 5:
the Queue interface

Java 5 introduces several queue implementations to the Collections framework. Queue implementations firstly share a new Queue interface, which has several methods for accessing the head and tail of the queue. Recall that items are in general always placed on the end or "tail" of the list, and always read from the beginning or "head" of the list. (An exception that we'll see is that in the case of priority queues, there's no actual "tail", although the notion still more or less applies.)

Methods specified by the Java Queue interface

Operation	Throws exception if not possible	Returns value if not possible
Add item to tail	add()	offer()
Remove item from head	remove()	poll()
"Peek" item at head	element()	peek()

Types of Queues

Java provides Queue implementations depending on a few key criteria:

• thread-safety: if you don't require the queue to be accessed concurrently from multiple threads, then a plain LinkedList can be used as a Queue; the advantage of various of the other implementations is that they offer efficient thread-safety;
• blocking or non-blocking: various blocking implementations add extra methods to put and remove items from the queue, blocking until the operation is possible, with an optional time limit;
• bound or non-bound: sometimes it is useful to put an upper limit on the number of items that can fit in the queue, e.g. to prevent a thread pool from queueing up too many jobs when the machine is busy;
• other special operations: Java provides an implementation that orders by priority, and another that applies a delay to queued items.

As of Java 6, the various queue classes are as follows:

Queue implementations as of Java 6

Blocking?	Other criteria	Bound	Non-bound
Blocking	None	ArrayBlockingQueue	LinkedBlockingQueue
	Priority-based		PriorityBlockingQueue
	Delayed		DelayQueue
Non-blocking	Thread-safe		ConcurrentLinkedQueue
	Non thread-safe		LinkedList
	Non thread-safe, priority-based		PriorityQueue

One further type of queue not included above is the SynchronousQueue, which is effectively a zero-length queue (so that a thread adding an item to the queue will block until another thread removes the item).

Blocking queues

In general, the most interesting queue implementations are the various blocking queues, which allow efficient concurrent access and are useful for coordinating objects between threads, particularly in the so-called producer-consumer pattern. In Java, all blocking queue implementations implement the BlockingQueue interface, which we look at next.

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