Queues typically, but do not necessarily order elements in a - * FIFO (first-in-first-out) manner. Among the exceptions are priority - * queues, that order elements in accord with supplied - * Comparators. Every Queue implementation must specify its ordering - * guarantees, - * - *
The offer method adds an element if possible, - * otherwise returning false. This differs from the - * Collections.add method, that throws an unchecked exception upon + * A collection designed for holding elements prior to processing. + * Besides basic {@link java.util.Collection 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 + * comparator, or the elements' natural ordering, and LIFO queues (or + * stacks) which order the elements LIFO (last-in-first-out). + * Whatever the ordering used, the head of the queue is that element + * which would be removed by a call to {@link #remove() } or {@link #poll()}. + * Every Queue implementation must specify its ordering guarantees. + * + *
The {@link #offer offer} method adds an element if possible, otherwise + * returning false. This differs from the + * {@link java.util.Collection#add Collection.add} + * 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 queues. + * in fixed-capacity (or "bounded") queues. * - *
The remove and poll methods delete and return - * an element in accord with the implementation's ordering policies -- - * for example, in FIFO queues, it will return the oldest element. - * The remove and poll differ only in their behavior - * when the queue is empty: poll returns null while - * remove throws an exception. These are designed for usage - * contexts in which emptiness is considered to be normal versus - * exceptional. - * - *
The element and peek methods return but do - * not delete the element that would be obtained by a call to - * remove and poll respectively. - * - *
The Queue interface does not define blocking queue methods - * (i.e., those that wait for elements to appear and/or for space to - * be available) that are common in concurrent programming. These are - * defined in the extended java.util.concurrent.BlockingQueue - * interface. - * - *
Queue implementations generally do not allow insertion of - * null. Even in those that allow it, it is a very bad idea - * to do so, since null is also used as a sentinel by - * poll to indicate that no elements exist. - **/ + *
The {@link #remove()} and {@link #poll()} methods remove and + * return the head of the queue. + * Exactly which element is removed from the queue is a + * function of the queue's ordering policy, which differs from + * implementation to implementation. 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 remove, the head of the queue. + * + *
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, although some implementations, such as + * {@link LinkedList}, do not prohibit insertion of null. + * Even in the implementations that permit it, null should + * not be inserted into a Queue, 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 java.util.Collection
+ * @see LinkedList
+ * @see PriorityQueue
+ * @see java.util.concurrent.LinkedBlockingQueue
+ * @see java.util.concurrent.BlockingQueue
+ * @see java.util.concurrent.ArrayBlockingQueue
+ * @see java.util.concurrent.LinkedBlockingQueue
+ * @see java.util.concurrent.PriorityBlockingQueue
+ * @since 1.5
+ * @author Doug Lea
+ */
public interface Queue