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From: Robert Klemme <shortcutter@googlemail.com>
Newsgroups: comp.lang.java.programmer
Subject: Re: The first 10 files
Date: Sun, 27 Jan 2013 13:55:18 +0100
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On 27.01.2013 03:43, Arne Vajh=F8j wrote:
> On 1/26/2013 9:35 PM, Arne Vajh=F8j wrote:
>> On 1/26/2013 9:02 PM, Arved Sandstrom wrote:

>> If OP happens to be on Java 7, then I will suggest using:
>>
>> java.nio.file.Files.newDirectoryStream(dir)
>>
>> It is a straight forward way of getting the first N files.
>>
>> And it is is as likely as the exception hack to not to read
>> all filenames from the OS.
>
> import java.io.IOException;
> import java.nio.file.Files;
> import java.nio.file.Path;
> import java.nio.file.Paths;
> import java.util.Iterator;
>
> public class ListFilesWithLimit {
>      public static void main(String[] args) throws IOException {
>          Iterator<Path> dir =3D
> Files.newDirectoryStream(Paths.get("/work")).iterator();
>          int n =3D 0;
>          while(dir.hasNext() && n < 10) {
>              System.out.println(dir.next());
>          }
>      }
> }

For earlier Java versions we could emulate that with a second thread.

package file;

import java.io.File;
import java.io.FileFilter;
import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.BlockingQueue;
import java.util.concurrent.SynchronousQueue;
import java.util.concurrent.TimeUnit;

public final class ListFileTestThreaded2 {

   private static final class CountFilterThread extends Thread=20
implements FileFilter {

     private final File dir;
     private final int maxFiles;
     private final BlockingQueue<List<File>> queue;
     private List<File> filesSeen =3D new ArrayList<File>();

     public CountFilterThread(File dir, int maxFiles,=20
BlockingQueue<List<File>> queue) {
       this.dir =3D dir;
       this.maxFiles =3D maxFiles;
       this.queue =3D queue;
     }

     @Override
     public void run() {
       try {
         dir.listFiles(this);

         if (filesSeen !=3D null) {
           send();
         }
       } catch (InterruptedException e) {
         e.printStackTrace();
       }
     }

     private void send() throws InterruptedException {
       queue.put(filesSeen);
       filesSeen =3D null;
     }

     @Override
     public boolean accept(final File f) {
       try {
         if (filesSeen !=3D null) {
           filesSeen.add(f);

           if (filesSeen.size() =3D=3D maxFiles) {
             send();
             assert filesSeen =3D=3D null;
           }
         }

         return false;
       } catch (InterruptedException e) {
         throw new IllegalStateException(e);
       }
     }
   }

   private static final int[] LIMITS =3D { 10, 100, 1000, 10000,=20
Integer.MAX_VALUE };

   public static void main(String[] args) throws InterruptedException {
     for (final String s : args) {
       System.out.println("Testing: " + s);
       final File dir =3D new File(s);

       if (dir.isDirectory()) {
         for (final int limit : LIMITS) {
           final SynchronousQueue<List<File>> queue =3D new=20
SynchronousQueue<List<File>>();
           final CountFilterThread cf =3D new CountFilterThread(dir,=20
limit, queue);
           cf.setDaemon(true);
           final long t1 =3D System.nanoTime();
           cf.start();
           final List<File> entries =3D queue.take();
           final long delta =3D System.nanoTime() - t1;
           System.out.printf("It took %20dus to retrieve %20d files,=20
%20.5fus/file.\n",
               TimeUnit.NANOSECONDS.toMicros(delta), entries.size(),=20
(double) TimeUnit.NANOSECONDS.toMicros(delta)
                   / entries.size());
         }
       } else {
         System.out.println("Not a directory.");
       }
     }

     System.out.println("done");
   }

}

https://gist.github.com/4648256

It's not guaranteed though that this will be faster.  And it's=20
definitively not simpler than the straight forward approach. :-)

Cheers

	robert



--=20
remember.guy do |as, often| as.you_can - without end
http://blog.rubybestpractices.com/