At Data Geekery, we love Java. And as we’re really into jOOQ’s fluent API and query DSL, we’re absolutely thrilled about what Java 8 will bring to our ecosystem. We have blogged a couple of times about some nice Java 8 goodies, and now we feel it’s time to start a new blog series, the…
Java 8 Friday
Every Friday, we’re showing you a couple of nice new tutorial-style Java 8 features, which take advantage of lambda expressions, extension methods, and other great stuff. You’ll find the source code on GitHub.
Interacting with the file system has been a bit of a pain in Java. CTMMC shows us an example of how to copy a file with Java. While some issues still remain, at least, we can now use lambdas and the new Streams API to traverse the file system and list files! Here is the FileFilterGoodies example that we’ve pushed to our GitHub repository:
public class FileFilterGoodies { public static void main(String args[]) { listRecursive(new File(".")); } /** * This method recursively lists all * .txt and .java files in a directory */ private static void listRecursive(File dir) { Arrays.stream(dir.listFiles((f, n) -> !n.startsWith(".") && (new File(f, n).isDirectory() || n.endsWith(".txt") || n.endsWith(".java")) )) .forEach(unchecked((file) -> { System.out.println( file.getCanonicalPath() .substring(new File(".") .getCanonicalPath() .length())); if (file.isDirectory()) { listRecursive(file); } })); } /** * This utility simply wraps a functional * interface that throws a checked exception * into a Java 8 Consumer */ private static <T> Consumer<T> unchecked(CheckedConsumer<T> consumer) { return t -> { try { consumer.accept(t); } catch (Exception e) { throw new RuntimeException(e); } }; } @FunctionalInterface private interface CheckedConsumer<T> { void accept(T t) throws Exception; } }
The output of the above programme is:
\LICENSE.txt \out \out\production \out\production\jOOQ's Java 8 Goodies \out\production\jOOQ's Java 8 Goodies\org \out\production\jOOQ's Java 8 Goodies\org\jooq \out\production\jOOQ's Java 8 Goodies\org\jooq\java8 \out\production\jOOQ's Java 8 Goodies\org\jooq\java8\goodies \out\production\jOOQ's Java 8 Goodies\org\jooq\java8\goodies\io \README.txt \src \src\org \src\org\jooq \src\org\jooq\java8 \src\org\jooq\java8\goodies \src\org\jooq\java8\goodies\io \src\org\jooq\java8\goodies\io\FileFilterGoodies.java
Now, that’s really awesome, isn’t it? Let’s decompose the above listRecursive()
method:
// With this method, we wrap the File[] array // into a new Java 8 Stream, which has awesome // new methods. Arrays.stream( // The Java 1.2 File.listFiles() method luckily // accepts a @FunctionalInterface, which can be // instantiated using a lambda expression // ... // In this example, we'll just ignore the fact // that listFiles can return null dir.listFiles((f, n) -> !n.startsWith(".") && (new File(f, n).isDirectory() || n.endsWith(".txt") || n.endsWith(".java")) )) // Each Stream (and also java.util.List) has this // awesome forEach method, that accepts a Consumer .forEach( // Unfortunately, Java 8 Consumers don't allow // throwing checked exceptions. So let's quickly // wrap it (see details below) ... unchecked( // ... and pass another lambda expression to it, // which prints the local path and recurses (file) -> { System.out.println( file.getCanonicalPath() .substring(new File(".") .getCanonicalPath() .length())); if (file.isDirectory()) { listRecursive(file); } }));
More goodies next week
Stay tuned for next week, when we show you how Java 8 improves using XML with jOOX
More on Java 8
In the mean time, have a look at Eugen Paraschiv’s awesome Java 8 resources page
A couple of comments:
1. File I/O was largely fixed in Java 7 with the new NIO.2 APIs including CTMMC #35 you referenced in this post.
2. Rather than using dir.listFiles() I’d use the new Files.newDirectoryStream() function which you can also give a lambda expression to apply filtering. The advantage here is that you’re building an Iterable rather than an Array so it scales to millions of files.
While it hasn’t been deprecated, java.io.File is considered legacy as of Java 7. They recommend using java.nio.file.Path instead.
Interesting post, really need to check Java 8 out.
I think your example could be improved a lot by changing `listRecursive` to return a sequence of all the files, this would enable the caller to decide what files she is interested in (filter) and what to do with them (map, each, fold).
Yes, it could be improved this way, of course. As others pointed out, though, Java 7 already greatly improved upon this particular API. I must admit, I haven’t had a look at those improvements yet. So there are chances that some things have improved even without using lambdas. And I suspect that there are new methods in that area for Java 8, e.g.
Files.walk()
: which returns a Stream:http://download.java.net/jdk8/docs/api/java/nio/file/Files.html#walk-java.nio.file.Path-java.nio.file.FileVisitOption…-
This will probably be the topic for another post :-)
n.endsWith(“.txt”)
the filter doesnt work. your code also lists *.class files etc.
Thank you very much for the hint. Fixed. The error was in checking whether
f.isDirectory()
, which is always true. It should benew File(f, n).isDirectory()