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<h1><i class="chapter">Chapter TWENTY-FIVE</i><br />
Files and Streams</h1>
<p><br /></p>
<h3 style="text-align: center;"><i>Exam Objectives</i></h3>
<p style="text-align: center;"><i>Use Stream API with NIO.2.</i><br /></p>
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<h2>New Stream Methods in NIO.2</h2>
<p>With the arrival of streams to Java, some difficult file operations to implement in NIO.2 (that often required an entire class) have been simplified.</p>
<p><code>java.nio.file.Files</code>, a class with static methods that we reviewed in the last chapter, added operations that return implementations of the <code>Stream</code> interface.<br /></p>
<p>The methods are:</p>
<p><code class="java hljs"><span class="hljs-function"><span class="hljs-keyword">static</span> Stream<Path> <span class="hljs-title">find</span><span class="hljs-params">(Path start,<br />
<span class="hljs-keyword"> int</span> maxDepth,<br />
BiPredicate<Path,<br />
BasicFileAttributes> matcher,<br />
FileVisitOption... options)</span><br />
<span class="hljs-keyword"><br />
static</span> Stream<Path> <span class="hljs-title">list</span><span class="hljs-params">(Path dir)</span><br />
<span class="hljs-keyword"><br />
static</span> Stream<String> <span class="hljs-title">lines</span><span class="hljs-params">(Path path)</span><br />
<span class="hljs-keyword">static</span> Stream<String> <span class="hljs-title">lines</span><span class="hljs-params">(Path path, Charset cs)</span><br />
<span class="hljs-keyword"><br />
static</span> Stream<Path> <span class="hljs-title">walk</span><span class="hljs-params">(Path start,<br />
FileVisitOption... options)</span><br />
<span class="hljs-keyword">static</span> Stream<Path> <span class="hljs-title">walk</span><span class="hljs-params">(Path start,<br />
<span class="hljs-keyword"> int</span> maxDepth,<br />
FileVisitOption... options)</span></span></code></p>
<p>An important thing to notice is that the returned streams are <b>LAZY</b>, which means that the elements are not loaded (or read) until they are used. This is a great performance enhancement.</p>
<p>Let's describe each method starting with <code>Files.list()</code>.</p>
<h2>Files.list()</h2>
<p><code class="java hljs"><span class="hljs-function"><span class="hljs-keyword">static</span> Stream<Path> <span class="hljs-title">list</span><span class="hljs-params">(Path dir)</span><br />
<span class="hljs-keyword"> throws</span> IOException</span></code></p>
<p>This method iterates over a directory to return a stream whose elements are <code>Path</code> objects that represent the entries of that directory.</p>
<p><code class="java hljs"><span class="hljs-keyword">try</span>(Stream<Path> stream =<br />
Files.list(Paths.get(<span class="hljs-string">"/temp"</span>))) {<br />
stream.forEach(System.out::println);<br />
} <span class="hljs-keyword">catch</span>(IOException e) {<br />
e.printStackTrace();<br />
}</code></p>
<p>A possible output:</p>
<p><code class="java hljs">/temp/dir1<br />
/temp/dir2<br />
/temp/file.txt</code></p>
<p>The use of a <code>try-with-resources</code> is recommended so the stream's close method can be invoked to close the file system resources.</p>
<p>As you can see, this method lists directories and files in the specified directory. However, it is not recursive, in other words, it <b>DOESN'T</b> traverse subdirectories.</p>
<p>Another two important considerations about this method:</p>
<ul>
<li>If the argument doesn't represent a directory, an exception is thrown.</li>
<li>This method is thread safe but is weakly consistent, meaning that while iterating a directory, updates to it may or may not be reflected in the returned stream.</li>
</ul>
<h2>Files.walk()</h2>
<p><code class="java hljs"><span class="hljs-function"><span class="hljs-keyword">static</span> Stream<Path> <span class="hljs-title">walk</span><span class="hljs-params">(Path start,<br />
FileVisitOption... options)</span><br />
<span class="hljs-keyword"> throws</span> IOException<br />
<span class="hljs-keyword">static</span> Stream<Path> <span class="hljs-title">walk</span><span class="hljs-params">(Path start,<br />
<span class="hljs-keyword"> int</span> maxDepth,<br />
FileVisitOption... options)</span><br />
<span class="hljs-keyword"> throws</span> IOException</span></code></p>
<p>This method also iterates over a directory to return a stream whose elements are <code>Path</code> objects that represent the entries of that directory.</p>
<p>The difference with <code>Files.list()</code> is that <code>Files.walk()</code> <b>DOES</b> recursively traverse the subdirectories.</p>
<p>It does it with a <i>depth-first</i> strategy, which traverses the directory structure from the root to all the way down a subdirectory before exploring another one.</p>
<p>For example, considering this structure:</p>
<p><code class="java hljs">/temp/<br />
/dir1/<br />
/subdir1/<br />
111.txt<br />
/subdir2/<br />
121.txt<br />
122.txt<br />
/dir2/<br />
21.txt<br />
22.txt<br />
file.txt</code></p>
<p>The following code:</p>
<p><code class="java hljs"><span class="hljs-keyword">try</span>(Stream<Path> stream =<br />
Files.walk(Paths.get(<span class="hljs-string">"c:/temp"</span>))) {<br />
stream.forEach(System.out::println);<br />
} <span class="hljs-keyword">catch</span>(IOException e) {<br />
e.printStackTrace();<br />
}</code></p>
<p>Will output:</p>
<p><code class="java hljs">/temp<br />
/temp/dir1<br />
/temp/dir1/subdir1<br />
/temp/dir1/subdir1/111.txt<br />
/temp/dir1/subdir2<br />
/temp/dir1/subdir2/121.txt<br />
/temp/dir1/subdir2/122.txt<br />
/temp/dir2<br />
/temp/dir2/21.txt<br />
/temp/dir2/22.txt<br />
/temp/file.txt</code></p>
<p>By default, this method uses a maximum subdirectory depth of <code>Integer.MAX_VALUE</code>. But you can use the overloaded version that takes the maximum depth as the second parameter:</p>
<p><code class="java hljs"><span class="hljs-keyword">try</span>(Stream<Path> stream =<br />
Files.walk(Paths.get(<span class="hljs-string">"/temp"</span>), <span class="hljs-number">1</span>)) {<br />
stream.forEach(System.out::println);<br />
} <span class="hljs-keyword">catch</span>(IOException e) {<br />
e.printStackTrace();<br />
}</code></p>
<p>The output is:</p>
<p><code class="java hljs">/temp<br />
/temp/dir1<br />
/temp/dir2<br />
/temp/file.txt</code></p>
<p>A value of <code>0</code> means that only the starting directory is visited.</p>
<p>Also, this method doesn't follow symbolic links by default.</p>
<p>Symbolic links can cause a <i>cycle</i>, an infinite circular dependency between directories. However, this method is smart enough to detect a cycle and throw a <code>FileSystemLoopException</code>.</p>
<p>To follow symbolic links, just use the argument of type <code>FileVisitOption</code> (preferably, also using the maximum depth argument) this way:</p>
<p><code class="java hljs"><span class="hljs-keyword">try</span>(Stream<Path> stream =<br />
Files.walk(Paths.get(<span class="hljs-string">"/temp"</span>),<br />
<span class="hljs-number"> 2</span>,<br />
FileVisitOption.FOLLOW_LINKS)<br />
) {<br />
stream.forEach(System.out::println);<br />
} <span class="hljs-keyword">catch</span>(IOException e) {<br />
e.printStackTrace();<br />
}</code></p>
<p>Just like <code>Files.list()</code>, it's recommended to use <code>Files.walk()</code> with <code>try-with-resources</code>, if the argument doesn't represent a directory, an exception is thrown and it's considered a <i>weakly consistent</i> method.</p>
<h2>Files.find()</h2>
<p><code class="java hljs"><span class="hljs-function"><span class="hljs-keyword">static</span> Stream<Path> <span class="hljs-title">find</span><span class="hljs-params">(Path start,<br />
<span class="hljs-keyword"> int</span> maxDepth,<br />
BiPredicate<Path,BasicFileAttributes> matcher,<br />
FileVisitOption... options)</span><br />
<span class="hljs-keyword"> throws</span> IOException</span></code></p>
<p>This method is similar to <code>Files.walk()</code>, but takes an additional argument of type <code>BiPredicate</code> that is used to filter the files and directories.</p>
<p>Remember that a <code>BiPredicate</code> takes two arguments and returns a <code>boolean</code>. In this case:</p>
<ul>
<li>The first argument is the <code>Path</code> object that represents the file or directory.</li>
<li>The second argument is a <code>BasicFileAttributes</code> object that represents the attributes of the file or directory in the file system (like creation time, if it's a file, directory or symbolic link, size, etc.).</li>
<li>The returned <code>boolean</code> indicates if the file should be included in the returned stream.</li>
</ul>
<p>The following example returns a stream that includes just directories:</p>
<p><code class="java hljs">BiPredicate<Path, BasicFileAttributes> predicate =<br />
(path, attrs) -> {<br />
<span class="hljs-keyword">return</span> attrs.isDirectory();<br />
};<br />
<span class="hljs-keyword">int</span> maxDepth = <span class="hljs-number">2</span>;<br />
<span class="hljs-keyword">try</span>(Stream<Path> stream =<br />
Files.find(Paths.get(<span class="hljs-string">"/temp"</span>),<br />
maxDepth, predicate)) {<br />
stream.forEach(System.out::println);<br />
} <span class="hljs-keyword">catch</span>(IOException e) {<br />
e.printStackTrace();<br />
}</code></p>
<p>A possible output can be:</p>
<p><code class="java hljs">/temp<br />
/temp/dir1<br />
/temp/dir1/subdir1<br />
/temp/dir1/subdir2<br />
/temp/dir2</code></p>
<p>Like <code>Files.walk()</code>, it can also take a <code>FileVisitOption</code> for visiting symbolic links, it's recommended to use it in a <code>try-with-resources</code> and throws an exception if it cannot read a file or directory.</p>
<h2>Files.lines()</h2>
<p><code class="java hljs"><span class="hljs-function"><span class="hljs-keyword">static</span> Stream<String> <span class="hljs-title">lines</span><span class="hljs-params">(Path path, Charset cs)</span><br />
<span class="hljs-keyword"> throws</span> IOException<br />
<span class="hljs-keyword">static</span> Stream<String> <span class="hljs-title">lines</span><span class="hljs-params">(Path path)</span><br />
<span class="hljs-keyword"> throws</span> IOException</span></code></p>
<p>This method reads all the lines of a file as a stream of <code>String</code>s.</p>
<p>As the stream is lazy, it doesn't load all the lines into memory, only the line read at any given time. If the file doesn't exist, an exception is thrown.</p>
<p>The file's bytes are decoded using the specified charset or with <code>UTF-8</code> by default.</p>
<p>For example:</p>
<p><code class="java hljs"><span class="hljs-keyword">try</span>(Stream<String> stream =<br />
Files.lines(Paths.get(<span class="hljs-string">"/temp/file.txt"</span>))) {<br />
stream.forEach(System.out::println);<br />
} <span class="hljs-keyword">catch</span>(IOException e) {<br />
e.printStackTrace();<br />
}</code></p>
<p>In Java 8, a <code>lines()</code> method was added to <code>java.io.BufferedReader</code> as well:</p>
<p><code class="java hljs"><span class="hljs-function">Stream<String> <span class="hljs-title">lines</span><span class="hljs-params">()</span></span></code></p>
<p>The stream is lazy and its elements are the lines read from the <code>BufferedReader</code>.</p>
<h2>Key Points</h2>
<ul>
<li>In Java 8, new methods that return implementations of the <code>Stream</code> interface have been added to <code>java.nio.file.Files</code>.</li>
<li>The returned streams are <b>LAZY</b>, which means that the elements are not loaded (or read) until they are used.</li>
<li>The use of a <code>try-with-resources</code> with these methods is recommended so that the stream's <code>close</code> method can be invoked to close the file system resources.</li>
<li><code>Files.list()</code> iterates over a directory to return a stream whose elements are <code>Path</code> objects that represent the entries of that directory.</li>
<li>This method lists directories and files of the specified directory. However, it is not recursive, in other words, it <b>DOESN'T</b> traverse subdirectories.</li>
<li><code>Files.walk()</code> also iterates over a directory in a depth-first strategy to return a stream whose elements are<code>Path</code> objects that represent the entries of that directory.</li>
<li>The difference with <code>Files.list()</code> is that <code>Files.walk()</code> <b>DOES</b> recursively traverse the subdirectories. You can also pass the maximum traversal depth and an option to follow symbolic links.</li>
<li><code>Files.find()</code> is similar to <code>Files.walk()</code>, but takes an additional argument of type <code>BiPredicate<Path,BasicFileAttributes></code> that is used to filter the files and directories.</li>
<li><code>Files.lines()</code> reads all the lines of a file as a stream of Strings without loading them all into memory.</li>
</ul>
<h2>Self Test</h2>
<p>1. Given the following structure and class:</p>
<p><code class="java hljs">/temp/<br />
/dir1/<br />
1.txt<br />
0.txt</code></p>
<p><code class="java hljs"><span class="hljs-keyword">public</span> <span class="hljs-class"><span class="hljs-keyword">class</span> <span class="hljs-title">Question_25_1</span></span> {<br />
<span class="hljs-function"><span class="hljs-keyword"> public</span> <span class="hljs-keyword">static</span> <span class="hljs-keyword">void</span> <span class="hljs-title">main</span><span class="hljs-params">(String[] args)</span></span> {<br />
<span class="hljs-keyword"> try</span>(Stream<Path> stream =<br />
Files.walk(Paths.get(<span class="hljs-string">"/temp"</span>), <span class="hljs-number">0</span>)) {<br />
stream.forEach(System.out::println);<br />
} <span class="hljs-keyword">catch</span>(IOException e) { }<br />
}<br />
}</code></p>
<p>What is the result?<br /> A. <code>/temp</code><br /> B. <code>/temp/dir1<br />
/temp/0.txt</code><br /> C. <code>/temp/0.txt</code><br /> D. Nothing is printed</p>
<p>2. Which of the following statements is true?<br /> A. <code>Files.find()</code> has a default subdirectory depth of <code>Integer.MAX_VALUE</code>.<br /> B. <code>Files.find()</code> follows symbolic links by default.<br /> C. <code>Files.walk()</code> follows symbolic links by default.<br /> D. <code>Files.walk()</code> traverses subdirectories recursively.</p>
<p>3. Which of the following options is equivalent to</p>
<p><code class="java hljs">Files.walk(Paths.get(<span class="hljs-string">"."</span>))<br />
.filter(p -> p.toString().endsWith(<span class="hljs-string">"txt"</span>));</code></p>
<p>A. <code class="hljs">Files.list(Paths.get("."))<br />
.filter(p -> p.toString().endsWith("txt"));</code><br /> B. <code class="hljs">Files.find(Paths.get("."),<br />
(p,a) -> p.toString().endsWith("txt"));</code><br /> C. <code class="hljs">Files.find(Paths.get("."), Integer.MAX_VALUE,<br />
p -> p.toString().endsWith("txt"));</code><br /> D. <code class="hljs">Files.find(Paths.get("."), Integer.MAX_VALUE,<br />
(p,a) -> p.toString().endsWith("txt"));</code></p>
<p>4. Which is the behavior of <code>Files.lines(Path)</code> if the <code>Path</code> object represents a file that doesn't exist?<br /> A. It returns an empty stream.<br /> B. It creates the file.<br /> C. It throws an <code>IOException</code> when the method is called.<br /> D. It throws an <code>IOException</code> when the stream is first used.</p>
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