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tim |
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package java.util; |
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/** |
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* Queues are Collections supporting additional basic insertion, |
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* extraction, and inspection operations. |
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* |
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* <p> Queues typically, but do not necessarily order elements in a |
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* FIFO (first-in-first-out) manner. Among the exceptions are priority |
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* queues, that order elements in accord with supplied |
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* Comparators. Every Queue implementation must specify its ordering |
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* guarantees, |
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* |
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* <p> The <tt>offer</tt> method adds an element if possible, |
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* otherwise returning <tt>false</tt>. This differs from the |
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* Collections.add method, that throws an unchecked exception upon |
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* failure. It is designed for use in collections in which failure to |
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* add is a normal, rather than exceptional occurrence, for example, |
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* in fixed-capacity queues. |
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* |
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* <p> The <tt>remove</tt> and <tt>poll</tt> methods delete and return |
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* an element in accord with the implementation's ordering policies -- |
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* for example, in FIFO queues, it will return the oldest element. |
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* The <tt>remove</tt> and <tt>poll</tt> differ only in their behavior |
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* when the queue is empty: <tt>poll</tt> returns <tt>null</tt> while |
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* <tt>remove</tt> throws an exception. These are designed for usage |
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* contexts in which emptiness is considered to be normal versus |
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* exceptional. |
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* |
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* <p> The <tt>element</tt> and <tt>peek</tt> methods return but do |
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* not delete the element that would be obtained by a call to |
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* <tt>remove</tt> and <tt>poll</tt> respectively. |
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* |
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* <p> The Queue interface does not define blocking queue methods |
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* (i.e., those that wait for elements to appear and/or for space to |
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* be available) that are common in concurrent programming. These are |
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* defined in the extended java.util.concurrent.BlockingQueue |
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* interface. |
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* |
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* <p> Queue implementations generally do not allow insertion of |
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* <tt>null</tt>. Even in those that allow it, it is a very bad idea |
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* to do so, since <tt>null</tt> is also used as a sentinel by |
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* <tt>poll</tt> to indicate that no elements exist. |
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**/ |
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public interface Queue<E> extends Collection<E> { |
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/** |
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* Add the given object to this queue if possible. |
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* @param x the object to add |
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* @return true if successful |
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**/ |
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public boolean offer(E x); |
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/** |
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* Delete and return an object from the queue if one is available. |
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* @return the object, or null if the queue is empty. |
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**/ |
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public E poll(); |
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/** |
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* Delete and return the element produced by poll, if the queue is |
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* not empty. |
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* @return an element |
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* @throws NoSuchElementException if empty |
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**/ |
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public E remove() throws NoSuchElementException; |
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/** |
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* Return but do not delete the element that will be returned by |
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* the next call to poll. |
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* @return an element, or null if empty |
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**/ |
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public E peek(); |
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/** |
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* Return but do not delete the element that will be returned by |
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* the next call to poll, if the queue is not empty. |
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* @return an element |
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* @throws NoSuchElementException if empty |
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**/ |
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public E element() throws NoSuchElementException; |
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} |
