| 1 |
package java.util; |
| 2 |
|
| 3 |
/** |
| 4 |
* Queues are Collections supporting additional basic insertion, |
| 5 |
* extraction, and inspection operations. |
| 6 |
* |
| 7 |
* <p> Queues typically, but do not necessarily order elements in a |
| 8 |
* FIFO (first-in-first-out) manner. Among the exceptions are priority |
| 9 |
* queues, that order elements in accord with supplied |
| 10 |
* Comparators. Every Queue implementation must specify its ordering |
| 11 |
* guarantees, |
| 12 |
* |
| 13 |
* <p> The <tt>offer</tt> method adds an element if possible, |
| 14 |
* otherwise returning <tt>false</tt>. This differs from the |
| 15 |
* Collections.add method, that throws an unchecked exception upon |
| 16 |
* failure. It is designed for use in collections in which failure to |
| 17 |
* add is a normal, rather than exceptional occurrence, for example, |
| 18 |
* in fixed-capacity queues. |
| 19 |
* |
| 20 |
* <p> The <tt>remove</tt> and <tt>poll</tt> methods delete and return |
| 21 |
* an element in accord with the implementation's ordering policies -- |
| 22 |
* for example, in FIFO queues, it will return the oldest element. |
| 23 |
* The <tt>remove</tt> and <tt>poll</tt> differ only in their behavior |
| 24 |
* when the queue is empty: <tt>poll</tt> returns <tt>null</tt> while |
| 25 |
* <tt>remove</tt> throws an exception. These are designed for usage |
| 26 |
* contexts in which emptiness is considered to be normal versus |
| 27 |
* exceptional. |
| 28 |
* |
| 29 |
* <p> The <tt>element</tt> and <tt>peek</tt> methods return but do |
| 30 |
* not delete the element that would be obtained by a call to |
| 31 |
* <tt>remove</tt> and <tt>poll</tt> respectively. |
| 32 |
* |
| 33 |
* <p> The Queue interface does not define blocking queue methods |
| 34 |
* (i.e., those that wait for elements to appear and/or for space to |
| 35 |
* be available) that are common in concurrent programming. These are |
| 36 |
* defined in the extended java.util.concurrent.BlockingQueue |
| 37 |
* interface. |
| 38 |
* |
| 39 |
* <p> Queue implementations generally do not allow insertion of |
| 40 |
* <tt>null</tt>. Even in those that allow it, it is a very bad idea |
| 41 |
* to do so, since <tt>null</tt> is also used as a sentinel by |
| 42 |
* <tt>poll</tt> to indicate that no elements exist. |
| 43 |
**/ |
| 44 |
public interface Queue<E> extends Collection<E> { |
| 45 |
|
| 46 |
/** |
| 47 |
* Add the given object to this queue if possible. |
| 48 |
* @param x the object to add |
| 49 |
* @return true if successful |
| 50 |
**/ |
| 51 |
public boolean offer(E x); |
| 52 |
|
| 53 |
/** |
| 54 |
* Delete and return an object from the queue if one is available. |
| 55 |
* @return the object, or null if the queue is empty. |
| 56 |
**/ |
| 57 |
public E poll(); |
| 58 |
|
| 59 |
/** |
| 60 |
* Delete and return the element produced by poll, if the queue is |
| 61 |
* not empty. |
| 62 |
* @return an element |
| 63 |
* @throws NoSuchElementException if empty |
| 64 |
**/ |
| 65 |
public E remove() throws NoSuchElementException; |
| 66 |
|
| 67 |
/** |
| 68 |
* Return but do not delete the element that will be returned by |
| 69 |
* the next call to poll. |
| 70 |
* @return an element, or null if empty |
| 71 |
**/ |
| 72 |
public E peek(); |
| 73 |
|
| 74 |
/** |
| 75 |
* Return but do not delete the element that will be returned by |
| 76 |
* the next call to poll, if the queue is not empty. |
| 77 |
* @return an element |
| 78 |
* @throws NoSuchElementException if empty |
| 79 |
**/ |
| 80 |
public E element() throws NoSuchElementException; |
| 81 |
} |
