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About Lukas Eder

Lukas is a Java and SQL enthusiast developer. He created the Data Geekery GmbH. He is the creator of jOOQ, a comprehensive SQL library for Java, and he is blogging mostly about these three topics: Java, SQL and jOOQ.

Java 8 Friday Goodies: java.io finally rocks!

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.

Java 8 Goodie: java.io with Lambdas

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(".")
                  &&
                     (f.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:

\jOOQ's Java 8 Goodies.iml
\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
\out\production\jOOQ's Java 8 Goodies\org\jooq\java8\goodies\io\FileFilterGoodies$CheckedConsumer.class
\out\production\jOOQ's Java 8 Goodies\org\jooq\java8\goodies\io\FileFilterGoodies.class
\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(".")
          &&
             (f.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
 

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One comment

  1. I hope it does rock, But the code example you gave here is hard to read. I’m suspicious about lambdas.They can easily make the code get hard to understand.
    thank you

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