Java/Perl - Directory handles

With Perl, one way to get a list of names in a given directory is to use a directory handle. On my Fedora box, Perl displays "." and ".." while Java does not.

Perl

#!/usr/bin/perl
use strict;
use warnings;

my $dir = '/home/kl93';
opendir DH, $dir or die "Cannot open $dir: $!";
foreach my  $file (readdir DH) {
  next if $file =~ /^\.\.?$/;     # skip . and ..
  print "$file\n";
}
close DH;

Java

import java.io.*;

public class ReadDir
{
  public static void main(String[] args) throws Exception
  {

    File file = new File("/home/kl93");

    if( !file.exists() || !file.canRead() )
    {
      System.out.println("Can't read " + file);
      return;
    }

    if( file.isDirectory() ) {
      String[] files = file.list();
      for(int i=0; i<files.length; i++)
        System.out.println(files[i]);
    }
    else {
      System.out.println(file + " is not a directory");
    }
  }
}

If you want to display filenames based on some sort of criteria - like all files with the .java extension for example - then it's often easiest to use a regular expression.

Perl

#!/usr/bin/perl
use strict;
use warnings;

my $dir = '/home/kl93';
opendir DH, $dir or die "Cannot open $dir: $!";
foreach my  $file (readdir DH) {
  next unless $file =~ /\.java$/;
  print "$file\n";
}
close DH;

Java

import java.io.*;
import java.util.regex.*;

public class ReadDir2
{
  public static void main(String[] args) throws Exception
  {

    File file = new File("/home/kl93");

    if( !file.exists() || !file.canRead() )
    {
      System.out.println("Can't read " + file);
      return;
    }

    if( file.isDirectory() ) {
      String[] files = file.list();
      for(int i=0; i<files.length; i++)
      {
        if(files[i].matches("^.*\\.java$"))
          System.out.println(files[i]);
      }
    }
    else {
      System.out.println(file + " is not a directory");
    }
  }
}

Or

import java.io.*;
import java.util.regex.*;

public class ReadDir3
{
  public static void main(String[] args) throws Exception
  {

    File file = new File("/home/kl93");

    if( !file.exists() || !file.canRead() )
    {
      System.out.println("Can't read " + file);
      return;
    }

    if( file.isDirectory() ) {
      String[] files = file.list();
      Pattern pattern = Pattern.compile("\\.java$");
      Matcher matcher;
      for(int i=0; i<files.length; i++)
      {
        matcher = pattern.matcher(files[i]);
        if(matcher.find())
          System.out.println(files[i]);
      }
    }
    else {
      System.out.println(file + " is not a directory");
    }
  }
}

Sort (ascending)

Perl

#!/usr/bin/perl
use strict;
use warnings;

my $dir = '/home/kl93';
opendir DH, $dir or die "Cannot open $dir: $!";
foreach my $file (sort readdir(DH)) {
  print "$file\n";
}
close DH;

Sort desending

#!/usr/bin/perl
use strict;
use warnings;

my $dir = '/home/kl93';
opendir DH, $dir or die "Cannot open $dir: $!";
foreach my $file (reverse(sort readdir(DH))) {
  print "$file\n";
}
close DH;