Working with Java Collections - Java in a Nutshell - page 253

Other orderings are also possible: a priority queue orders its elements according to
an external Comparator object, or according to the natural ordering of Comparable
elements. Unlike a Set , Queue implementations typically allow duplicate elements.
Unlike List , the Queue interface does not define methods for manipulating queue
elements at arbitrary positions. Only the element at the head of the queue is avail‐
able for examination. It is common for Queue implementations to have a fixed
capacity: when a queue is full, it is not possible to add more elements. Similarly,
when a queue is empty, it is not possible to remove any more elements. Because full
and empty conditions are a normal part of many queue-based algorithms, the Queue
interface defines methods that signal these conditions with return values rather than
by throwing exceptions. Specifically, the peek() and poll() methods return null to
indicate that the queue is empty. For this reason, most Queue implementations do
not allow null elements.

A blocking queue is a type of queue that defines blocking put() and take() meth‐
ods. The put() method adds an element to the queue, waiting, if necessary, until
there is space in the queue for the element. And the take() method removes an ele‐
ment from the head of the queue, waiting, if necessary, until there is an element to
remove. Blocking queues are an important part of many multithreaded algorithms,
and the BlockingQueue interface (which extends Queue ) is defined as part of the
java.util.concurrent package.

Queues are not nearly as commonly used as sets, lists, and maps, except perhaps in
certain multithreaded programming styles. In lieu of example code here, we'll try to
clarify the different possible queue insertion and removal operations.

Adding elements to queues

add()
This Collection method simply adds an element in the normal way. In boun‐
ded queues, this method may throw an exception if the queue is full.

offer()
This Queue method is like add() but returns false instead of throwing an
exception if the element cannot be added because a bounded queue is full.

BlockingQueue defines a timeout version of offer() that waits up to a speci‐
fied amount of time for space to become available in a full queue. Like the basic
version of the method, it returns true if the element was inserted and false
otherwise.

s

a

put( This BlockingQueue method blocks: if the element cannot be inserted because
the queue is full, put() waits until some other thread removes an element from
the queue, and space becomes available for the new element.