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 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.

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 * under the terms of the GNU General Public License version 2 only, as

 * published by the Free Software Foundation.  Oracle designates this

 * particular file as subject to the "Classpath" exception as provided

 * by Oracle in the LICENSE file that accompanied this code.

 *

 * This code is distributed in the hope that it will be useful, but WITHOUT

 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or

 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License

 * version 2 for more details (a copy is included in the LICENSE file that

 * accompanied this code).

 *

 * You should have received a copy of the GNU General Public License version

 * 2 along with this work; if not, write to the Free Software Foundation,

 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.

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package java.nio.file;

import java.util.List;

/**

 * A token representing the registration of a {@link Watchable watchable} object

 * with a {@link WatchService}.

 *

 * <p> A watch key is created when a watchable object is registered with a watch

 * service. The key remains {@link #isValid valid} until:

 * <ol>

 *   <li> It is cancelled, explicitly, by invoking its {@link #cancel cancel}

 *     method, or</li>

 *   <li> Cancelled implicitly, because the object is no longer accessible,

 *     or </li>

 *   <li> By {@link WatchService#close closing} the watch service. </li>

 * </ol>

 *

 * <p> A watch key has a state. When initially created the key is said to be

 * <em>ready</em>. When an event is detected then the key is <em>signalled</em>

 * and queued so that it can be retrieved by invoking the watch service's {@link

 * WatchService#poll() poll} or {@link WatchService#take() take} methods. Once

 * signalled, a key remains in this state until its {@link #reset reset} method

 * is invoked to return the key to the ready state. Events detected while the

 * key is in the signalled state are queued but do not cause the key to be

 * re-queued for retrieval from the watch service. Events are retrieved by

 * invoking the key's {@link #pollEvents pollEvents} method. This method

 * retrieves and removes all events accumulated for the object. When initially

 * created, a watch key has no pending events. Typically events are retrieved

 * when the key is in the signalled state leading to the following idiom:

 *

 * <pre>

 *     for (;;) {

 *         // retrieve key

 *         WatchKey key = watcher.take();

 *

 *         // process events

 *         for (WatchEvent<?> event: key.pollEvents()) {

 *             :

 *         }

 *

 *         // reset the key

 *         boolean valid = key.reset();

 *         if (!valid) {

 *             // object no longer registered

 *         }

 *     }

 * </pre>

 *

 * <p> Watch keys are safe for use by multiple concurrent threads. Where there

 * are several threads retrieving signalled keys from a watch service then care

 * should be taken to ensure that the {@code reset} method is only invoked after

 * the events for the object have been processed. This ensures that one thread

 * is processing the events for an object at any time.

 *

 * @since 1.7

 */

public interface WatchKey {

    /**

     * Tells whether or not this watch key is valid.

     *

     * <p> A watch key is valid upon creation and remains until it is cancelled,

     * or its watch service is closed.

     *

     * @return  {@code true} if, and only if, this watch key is valid

     */

    boolean isValid();

    /**

     * Retrieves and removes all pending events for this watch key, returning

     * a {@code List} of the events that were retrieved.

     *

     * <p> Note that this method does not wait if there are no events pending.

     *

     * @return  the list of the events retrieved; may be empty

     */

    List<WatchEvent<?>> pollEvents();

    /**

     * Resets this watch key.

     *

     * <p> If this watch key has been cancelled or this watch key is already in

     * the ready state then invoking this method has no effect. Otherwise

     * if there are pending events for the object then this watch key is

     * immediately re-queued to the watch service. If there are no pending

     * events then the watch key is put into the ready state and will remain in

     * that state until an event is detected or the watch key is cancelled.

     *

     * @return  {@code true} if the watch key is valid and has been reset, and

     *          {@code false} if the watch key could not be reset because it is

     *          no longer {@link #isValid valid}

     */

    boolean reset();

    /**

     * Cancels the registration with the watch service. Upon return the watch key

     * will be invalid. If the watch key is enqueued, waiting to be retrieved

     * from the watch service, then it will remain in the queue until it is

     * removed. Pending events, if any, remain pending and may be retrieved by

     * invoking the {@link #pollEvents pollEvents} method after the key is

     * cancelled.

     *

     * <p> If this watch key has already been cancelled then invoking this

     * method has no effect.  Once cancelled, a watch key remains forever invalid.

     */

    void cancel();

    /**

     * Returns the object for which this watch key was created. This method will

     * continue to return the object even after the key is cancelled.

     *

     * <p> As the {@code WatchService} is intended to map directly on to the

     * native file event notification facility (where available) then many of

     * details on how registered objects are watched is highly implementation

     * specific. When watching a directory for changes for example, and the

     * directory is moved or renamed in the file system, there is no guarantee

     * that the watch key will be cancelled and so the object returned by this

     * method may no longer be a valid path to the directory.

     *

     * @return the object for which this watch key was created

     */

    Watchable watchable();

}