/*

 * Copyright 2007-2009 Sun Microsystems, Inc.  All Rights Reserved.

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

 *

 * This code is free software; you can redistribute it and/or modify it

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

 * published by the Free Software Foundation.  Sun designates this

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

 * by Sun 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.

 *

 * Please contact Sun Microsystems, Inc., 4150 Network Circle, Santa Clara,

 * CA 95054 USA or visit www.sun.com if you need additional information or

 * have any questions.

 */

package java.nio.file;

import java.util.Iterator;

import java.io.Closeable;

/**

 * An object to iterate over the entries in a directory. A directory stream

 * allows for convenient use of the for-each construct:

 * <pre>

 *   Path dir = ...

 *   DirectoryStream<Path> stream = dir.newDirectoryStream();

 *   try {

 *       for (Path entry: stream) {

 *         ..

 *       }

 *   } finally {

 *       stream.close();

 *   }

 * </pre>

 *

 * <p><b> A {@code DirectoryStream} is not a general-purpose {@code Iterable}.

 * While this interface extends {@code Iterable}, the {@code iterator} method

 * may only be invoked once to obtain the iterator; a second, or subsequent,

 * call to the {@code iterator} method throws {@code IllegalStateException}. </b>

 *

 * <p> A {@code DirectoryStream} is opened upon creation and is closed by

 * invoking the {@link #close close} method. Closing the directory stream

 * releases any resources associated with the stream. The {@link

 * Files#withDirectory Files.withDirectory} utility method is useful for cases

 * where a task is performed on entries in a directory. This method automatically

 * closes the directory stream when iteration is complete (or an error occurs).

 * Once a directory stream is closed, all further method invocations on the

 * iterator throw {@link java.util.concurrent.ConcurrentModificationException}

 * with cause {@link ClosedDirectoryStreamException}.

 *

 * <p> A directory stream is not required to be <i>asynchronously closeable</i>.

 * If a thread is blocked on the directory stream's iterator, reading from the

 * directory, and another thread invokes the {@code close} method then it may

 * require to block until the read operation is complete.

 *

 * <p> The {@link Iterator#hasNext() hasNext} and {@link Iterator#next() next}

 * methods can encounter an I/O error when iterating over the directory in which

 * case {@code ConcurrentModificationException} is thrown with cause

 * {@link java.io.IOException}. The {@code hasNext} method is guaranteed to

 * read-ahead by at least one element. This means that if the {@code hasNext}

 * method returns {@code true} and is followed by a call to the {@code next}

 * method then it is guaranteed not to fail with a {@code

 * ConcurrentModificationException}.

 *

 * <p> The elements returned by the iterator are in no specific order. Some file

 * systems maintain special links to the directory itself and the directory's

 * parent directory. Entries representing these links are not returned by the

 * iterator.

 *

 * <p> The iterator's {@link Iterator#remove() remove} method removes the

 * directory entry for the last element returned by the iterator, as if by

 * invoking the {@link FileRef#delete delete} method. If an I/O error or

 * security exception occurs then {@code ConcurrentModificationException} is

 * thrown with the cause.

 *

 * <p> The iterator is <i>weakly consistent</i>. It is thread safe but does not

 * freeze the directory while iterating, so it may (or may not) reflect updates

 * to the directory that occur after the {@code DirectoryStream} is created.

 *

 * @param   <T>     The type of element returned by the iterator

 *

 * @since 1.7

 *

 * @see Path#newDirectoryStream

 */

public interface DirectoryStream<T>

    extends Closeable, Iterable<T>

{

    /**

     * An interface that is implemented by objects that decide if a directory

     * entry should be accepted or filtered. A {@code Filter} is passed as the

     * parameter to the {@link Path#newDirectoryStream(DirectoryStream.Filter)

     * newDirectoryStream} method when opening a directory to iterate over the

     * entries in the directory.

     *

     * <p> The {@link DirectoryStreamFilters} class defines factory methods to

     * create filters for a number of common usages and also methods to combine

     * filters.

     *

     * @param   <T>     The type of the directory entry

     *

     * @since 1.7

     */

    public static interface Filter<T> {

        /**

         * Decides if the given directory entry should be accepted or filtered.

         *

         * @param   entry

         *          The directory entry to be tested

         *

         * @return  {@code true} if the directory entry should be accepted

         */

        boolean accept(T entry);

    }

    /**

     * Returns the iterator associated with this {@code DirectoryStream}.

     *

     * @return  The iterator associated with this {@code DirectoryStream}

     *

     * @throws  IllegalStateException

     *          If this directory stream is closed or the iterator has already

     *          been returned

     */

    @Override

    Iterator<T> iterator();

}