package java.util; /** * A Collection designed for holding elements prior to processing. * Besides basic {@link Collection} operations, queues provide * additional insertion, extraction, and inspection operations. * *
Queues typically, but do not necessarily, order elements in a * FIFO (first-in-first-out) manner. Among the exceptions are priority * queues, which order elements according to a supplied comparators, or * the elements natural ordering. Every Queue implementation must specify * its ordering guarantees. * *
The {@link #offer(E)} method adds an element if possible, otherwise * returning false. This differs from the {@link * Collections#add(Object)} method, which throws an unchecked exception upon * failure. It is designed for use in collections in which failure to * add is a normal, rather than exceptional occurrence, for example, * in fixed-capacity (or “bounded”) queues. * *
The {@link #remove()} and {@link #poll()} methods remove and return an * element in accord with the implementation's ordering policy. For example, * in FIFO queues, they remove and return the oldest element in the queue. * The remove() and poll() methods differ only in their * behavior when the queue is empty: the remove() method throws an * exception, while the poll() method returns null. * *
The {@link #element()} and {@link #peek()} methods return but do * not delete the element that would be obtained by a call to * the remove and poll methods respectively. * *
The Queue interface does not define the blocking queue * methods, which are common in concurrent programming. These methods, * which wait for elements to appear or for space to become available, are * defined in the {@link java.util.concurrent.BlockingQueue} interface, which * extends this interface. * *
Queue implementations generally do not allow insertion of * null elements. Even in the few implementations that permit it, * it is a bad idea, as null is also used as a special return value * by the poll method to indicate that the queue contains no * elements. * *
This interface is a member of the
*
* Java Collections Framework.
*
* @see Collection
* @see LinkedList
* @see PriorityQueue
* @see LinkedQueue
* @see java.util.concurrent.BlockingQueue
* @see java.util.concurrent.ArrayBlockingQueue
* @see java.util.concurrent.LinkedBlockingQueue
* @see java.util.concurrent.PriorityBlockingQueue
*/
public interface Queue