1/*
2 * Copyright (c) 2007, 2013, Oracle and/or its affiliates. All rights reserved.
3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
4 *
5 * This code is free software; you can redistribute it and/or modify it
6 * under the terms of the GNU General Public License version 2 only, as
7 * published by the Free Software Foundation.  Oracle designates this
8 * particular file as subject to the "Classpath" exception as provided
9 * by Oracle in the LICENSE file that accompanied this code.
10 *
11 * This code is distributed in the hope that it will be useful, but WITHOUT
12 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
13 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
14 * version 2 for more details (a copy is included in the LICENSE file that
15 * accompanied this code).
16 *
17 * You should have received a copy of the GNU General Public License version
18 * 2 along with this work; if not, write to the Free Software Foundation,
19 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
20 *
21 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
22 * or visit www.oracle.com if you need additional information or have any
23 * questions.
24 */
25
26package java.nio.file;
27
28import java.util.Iterator;
29import java.io.Closeable;
30import java.io.IOException;
31
32/**
33 * An object to iterate over the entries in a directory. A directory stream
34 * allows for the convenient use of the for-each construct to iterate over a
35 * directory.
36 *
37 * <p> <b> While {@code DirectoryStream} extends {@code Iterable}, it is not a
38 * general-purpose {@code Iterable} as it supports only a single {@code
39 * Iterator}; invoking the {@link #iterator iterator} method to obtain a second
40 * or subsequent iterator throws {@code IllegalStateException}. </b>
41 *
42 * <p> An important property of the directory stream's {@code Iterator} is that
43 * its {@link Iterator#hasNext() hasNext} method is guaranteed to read-ahead by
44 * at least one element. If {@code hasNext} method returns {@code true}, and is
45 * followed by a call to the {@code next} method, it is guaranteed that the
46 * {@code next} method will not throw an exception due to an I/O error, or
47 * because the stream has been {@link #close closed}. The {@code Iterator} does
48 * not support the {@link Iterator#remove remove} operation.
49 *
50 * <p> A {@code DirectoryStream} is opened upon creation and is closed by
51 * invoking the {@code close} method. Closing a directory stream releases any
52 * resources associated with the stream. Failure to close the stream may result
53 * in a resource leak. The try-with-resources statement provides a useful
54 * construct to ensure that the stream is closed:
55 * <pre>
56 *   Path dir = ...
57 *   try (DirectoryStream&lt;Path&gt; stream = Files.newDirectoryStream(dir)) {
58 *       for (Path entry: stream) {
59 *           ...
60 *       }
61 *   }
62 * </pre>
63 *
64 * <p> Once a directory stream is closed, then further access to the directory,
65 * using the {@code Iterator}, behaves as if the end of stream has been reached.
66 * Due to read-ahead, the {@code Iterator} may return one or more elements
67 * after the directory stream has been closed. Once these buffered elements
68 * have been read, then subsequent calls to the {@code hasNext} method returns
69 * {@code false}, and subsequent calls to the {@code next} method will throw
70 * {@code NoSuchElementException}.
71 *
72 * <p> A directory stream is not required to be <i>asynchronously closeable</i>.
73 * If a thread is blocked on the directory stream's iterator reading from the
74 * directory, and another thread invokes the {@code close} method, then the
75 * second thread may block until the read operation is complete.
76 *
77 * <p> If an I/O error is encountered when accessing the directory then it
78 * causes the {@code Iterator}'s {@code hasNext} or {@code next} methods to
79 * throw {@link DirectoryIteratorException} with the {@link IOException} as the
80 * cause. As stated above, the {@code hasNext} method is guaranteed to
81 * read-ahead by at least one element. This means that if {@code hasNext} method
82 * returns {@code true}, and is followed by a call to the {@code next} method,
83 * then it is guaranteed that the {@code next} method will not fail with a
84 * {@code DirectoryIteratorException}.
85 *
86 * <p> The elements returned by the iterator are in no specific order. Some file
87 * systems maintain special links to the directory itself and the directory's
88 * parent directory. Entries representing these links are not returned by the
89 * iterator.
90 *
91 * <p> The iterator is <i>weakly consistent</i>. It is thread safe but does not
92 * freeze the directory while iterating, so it may (or may not) reflect updates
93 * to the directory that occur after the {@code DirectoryStream} is created.
94 *
95 * <p> <b>Usage Examples:</b>
96 * Suppose we want a list of the source files in a directory. This example uses
97 * both the for-each and try-with-resources constructs.
98 * <pre>
99 *   List&lt;Path&gt; listSourceFiles(Path dir) throws IOException {
100 *       List&lt;Path&gt; result = new ArrayList&lt;&gt;();
101 *       try (DirectoryStream&lt;Path&gt; stream = Files.newDirectoryStream(dir, "*.{c,h,cpp,hpp,java}")) {
102 *           for (Path entry: stream) {
103 *               result.add(entry);
104 *           }
105 *       } catch (DirectoryIteratorException ex) {
106 *           // I/O error encounted during the iteration, the cause is an IOException
107 *           throw ex.getCause();
108 *       }
109 *       return result;
110 *   }
111 * </pre>
112 * @param   <T>     The type of element returned by the iterator
113 *
114 * @since 1.7
115 *
116 * @see Files#newDirectoryStream(Path)
117 */
118
119public interface DirectoryStream<T>
120    extends Closeable, Iterable<T> {
121    /**
122     * An interface that is implemented by objects that decide if a directory
123     * entry should be accepted or filtered. A {@code Filter} is passed as the
124     * parameter to the {@link Files#newDirectoryStream(Path,DirectoryStream.Filter)}
125     * method when opening a directory to iterate over the entries in the
126     * directory.
127     *
128     * @param   <T>     the type of the directory entry
129     *
130     * @since 1.7
131     */
132    @FunctionalInterface
133    public static interface Filter<T> {
134        /**
135         * Decides if the given directory entry should be accepted or filtered.
136         *
137         * @param   entry
138         *          the directory entry to be tested
139         *
140         * @return  {@code true} if the directory entry should be accepted
141         *
142         * @throws  IOException
143         *          If an I/O error occurs
144         */
145        boolean accept(T entry) throws IOException;
146    }
147
148    /**
149     * Returns the iterator associated with this {@code DirectoryStream}.
150     *
151     * @return  the iterator associated with this {@code DirectoryStream}
152     *
153     * @throws  IllegalStateException
154     *          if this directory stream is closed or the iterator has already
155     *          been returned
156     */
157    @Override
158    Iterator<T> iterator();
159}
160