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* Copyright (c) 2007, 2013, Oracle and/or its affiliates. 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. 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.
*
* Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
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*/
package java.nio.file;
import java.util.Iterator;
import java.io.Closeable;
import java.io.IOException;
/**
* An object to iterate over the entries in a directory. A directory stream
* allows for the convenient use of the for-each construct to iterate over a
* directory.
*
* <p> <b> While {@code DirectoryStream} extends {@code Iterable}, it is not a
* general-purpose {@code Iterable} as it supports only a single {@code
* Iterator}; invoking the {@link #iterator iterator} method to obtain a second
* or subsequent iterator throws {@code IllegalStateException}. </b>
*
* <p> An important property of the directory stream's {@code Iterator} is that
* its {@link Iterator#hasNext() hasNext} method is guaranteed to read-ahead by
* at least one element. If {@code hasNext} method returns {@code true}, and is
* followed by a call to the {@code next} method, it is guaranteed that the
* {@code next} method will not throw an exception due to an I/O error, or
* because the stream has been {@link #close closed}. The {@code Iterator} does
* not support the {@link Iterator#remove remove} operation.
*
* <p> A {@code DirectoryStream} is opened upon creation and is closed by
* invoking the {@code close} method. Closing a directory stream releases any
* resources associated with the stream. Failure to close the stream may result
* in a resource leak. The try-with-resources statement provides a useful
* construct to ensure that the stream is closed:
* <pre>
* Path dir = ...
* try (DirectoryStream<Path> stream = Files.newDirectoryStream(dir)) {
* for (Path entry: stream) {
* ...
* }
* }
* </pre>
*
* <p> Once a directory stream is closed, then further access to the directory,
* using the {@code Iterator}, behaves as if the end of stream has been reached.
* Due to read-ahead, the {@code Iterator} may return one or more elements
* after the directory stream has been closed. Once these buffered elements
* have been read, then subsequent calls to the {@code hasNext} method returns
* {@code false}, and subsequent calls to the {@code next} method will throw
* {@code NoSuchElementException}.
*
* <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 the
* second thread may block until the read operation is complete.
*
* <p> If an I/O error is encountered when accessing the directory then it
* causes the {@code Iterator}'s {@code hasNext} or {@code next} methods to
* throw {@link DirectoryIteratorException} with the {@link IOException} as the
* cause. As stated above, the {@code hasNext} method is guaranteed to
* read-ahead by at least one element. This means that if {@code hasNext} method
* returns {@code true}, and is followed by a call to the {@code next} method,
* then it is guaranteed that the {@code next} method will not fail with a
* {@code DirectoryIteratorException}.
*
* <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 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.
*
* <p> <b>Usage Examples:</b>
* Suppose we want a list of the source files in a directory. This example uses
* both the for-each and try-with-resources constructs.
* <pre>
* List<Path> listSourceFiles(Path dir) throws IOException {
* List<Path> result = new ArrayList<>();
* try (DirectoryStream<Path> stream = Files.newDirectoryStream(dir, "*.{c,h,cpp,hpp,java}")) {
* for (Path entry: stream) {
* result.add(entry);
* }
* } catch (DirectoryIteratorException ex) {
* // I/O error encounted during the iteration, the cause is an IOException
* throw ex.getCause();
* }
* return result;
* }
* </pre>
* @param <T> The type of element returned by the iterator
*
* @since 1.7
*
* @see Files#newDirectoryStream(Path)
*/
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 Files#newDirectoryStream(Path,DirectoryStream.Filter)}
* method when opening a directory to iterate over the entries in the
* directory.
*
* @param <T> the type of the directory entry
*
* @since 1.7
*/
@FunctionalInterface
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
*
* @throws IOException
* If an I/O error occurs
*/
boolean accept(T entry) throws IOException;
}
/**
* 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();
}