List files with regular expression filtering

Byron KiourtzoglouNovember 11th, 2012Last Updated: August 19th, 2013

0 468 1 minute read

This is an example of how to list files using regular expression filtering. In order to list Files using regular expressions to filter them we have set the example below:

Class DirFilter implements the FilenameFilter. It has a Pattern that is initialized by compiling a String regular expression to a Pattern, using compile(String regex) API method of Pattern. It also has a method boolean accept(File dir, String name). The method reads a file name and creates a new File, and then gets the file name, using getName() API method. It creates a new Matcher from the pattern and the given String with matcher(CharSequence input) API method of Matcher and tries to match the entire region against the pattern, with matches() API method of Matcher.
Class AlphabeticComparator implements the Comparator and has a method, int compare(Object o1, Object o2). The method gets two objects and returns true if the String representation of the objects converted to characters in lower case are equal, according to the compareTo(String anotherString) API method of String.
We create a new File and get the array of strings naming the files and directories in the directory denoted by a new DirFilter initialized with a specified pattern.
We use sort(T[], Comparator<? super T>) API method of Arrays to sort the array according to the AlphabeticComparator and print the results.

Let’s take a look at the code snippet that follows:

package com.javacodegeeks.snippets.core;

import java.io.File;
import java.io.FilenameFilter;
import java.util.Arrays;
import java.util.Comparator;
import java.util.regex.Pattern;


public class DirList {
    
  public static void main(String[] args) {
    File filePath = new File(".");
    String[] list;
    list = filePath.list(new DirFilter("bu.+"));
    Arrays.sort(list, new AlphabeticComparator());
    for (int i = 0; i < list.length; i++)

System.out.println(list[i]);
  }
}


class DirFilter implements FilenameFilter {
  private Pattern pattern;

  public DirFilter(String reg) {
    pattern = Pattern.compile(reg);
  }

    @Override
  public boolean accept(File dir, String name) {
    // Strip path information, search for regex:
    return pattern.matcher(new File(name).getName()).matches();
  }
}

class AlphabeticComparator implements Comparator {
    @Override
  public int compare(Object o1, Object o2) {
    String str1 = (String) o1;
    String str2 = (String) o2;
    return str1.toLowerCase().compareTo(str2.toLowerCase());
  }
}

Output: