import java.io.*;
import java.util.Arrays;
import java.util.ArrayList;
import java.util.List;
import java.util.Comparator;

public class FileUtils{
  
  /**
   * returns a folder content sorted by extension
   * case-insensitive ,ascending order
   * subfolders are not included
   * @param folder  folder name
   * @return  
   * @throws IOException  if not a folder or not found
   */
  @SuppressWarnings("unchecked")
  public static File[] dirListByExtension(File folder) throws IOException {
    if (!folder.isDirectory() || !folder.exists()) {
      throw new IOException( folder.getName() + " : Not a folder or not exist");
    }
    File files[] = FileUtils.list(folder, false); // don't include subfolder
    Arrays.sort(files, new Comparator() {
      public int compare(final Object o1, final Object o2) {
          String s1 = ((File) o1).getName().toLowerCase();
          String s2 = ((File) o2).getName().toLowerCase();
          final int s1Dot = s1.lastIndexOf('.');
          final int s2Dot = s2.lastIndexOf('.');
          // 
          if ((s1Dot == -1) == (s2Dot == -1)) { // both or neither
              s1 = s1.substring(s1Dot + 1);
              s2 = s2.substring(s2Dot + 1);
              return s1.compareTo(s2);
          } else if (s1Dot == -1) { // only s2 has an extension, so s1 goes first
              return -1;
          } else { // only s1 has an extension, so s1 goes second
              return 1;
          }
      }
    });
   return files;
  }  
  
  /**
   * returns the content of a folder with subfolders included or not
   * @param folder   folder name
   * @param includeSubFolder  true/false
   * @return  array of files
   */
  public static File[] list(File folder, boolean includeSubFolder) {
    if (!folder.isDirectory()) {
      return null;
    }
    File files[] = folder.listFiles();
    List<File> list = new ArrayList<File>();
    for (File file : files) {
      if (file.isDirectory()) {
        if (includeSubFolder) {
          list.add(file);  
        }
      }
      else {
       list.add(file);
      }
    }
    if (list.isEmpty()) {
      return null;
    }
    return list.toArray(new File[]{});
  }

  /*
   * TEST
   */
  public static void main(String args[]) throws Exception {
    File files[] = FileUtils.dirListByExtension(new File("C:/temp"));
    for (File file : files) {
      System.out.println(file.getName());
    }
    // to reverse
    //   List<File> temp = Arrays.asList(files);
    //   java.util.Collections.reverse(temp);
    //   files = temp.toArray(new File[]{});
    //   for (File file : files) {
    ///     System.out.println(file.getName());
    //   }
  
  }

}