import java.util.*;import java.io.*;/*** Recursive file listing under a specified directory.* * @author javapractices.com* @author Alex Wong* @author anonymous user*/public final class FileListing { /** * Demonstrate use. *  * @param aArgs - <tt>aArgs[0]</tt> is the full name of an existing  * directory that can be read. */ public static void main(String... aArgs) throws FileNotFoundException {  File startingDirectory= new File(aArgs[0]);  List<File> files = FileListing.getFileListing(startingDirectory);  //print out all file names, in the the order of File.compareTo()  for(File file : files ){   System.out.println(file);  } } /** * Recursively walk a directory tree and return a List of all * Files found; the List is sorted using File.compareTo(). * * @param aStartingDir is a valid directory, which can be read. */ static public List<File> getFileListing(  File aStartingDir ) throws FileNotFoundException {  validateDirectory(aStartingDir);  List<File> result = getFileListingNoSort(aStartingDir);  Collections.sort(result);  return result; } // PRIVATE // static private List<File> getFileListingNoSort(  File aStartingDir ) throws FileNotFoundException {  List<File> result = new ArrayList<File>();  File[] filesAndDirs = aStartingDir.listFiles();  List<File> filesDirs = Arrays.asList(filesAndDirs);  for(File file : filesDirs) {   result.add(file); //always add, even if directory   if ( ! file.isFile() ) {    //must be a directory    //recursive call!    List<File> deeperList = getFileListingNoSort(file);    result.addAll(deeperList);   }  }  return result; } /** * Directory is valid if it exists, does not represent a file, and can be read. */ static private void validateDirectory (  File aDirectory ) throws FileNotFoundException {  if (aDirectory == null) {   throw new IllegalArgumentException("Directory should not be null.");  }  if (!aDirectory.exists()) {   throw new FileNotFoundException("Directory does not exist: " + aDirectory);  }  if (!aDirectory.isDirectory()) {   throw new IllegalArgumentException("Is not a directory: " + aDirectory);  }  if (!aDirectory.canRead()) {   throw new IllegalArgumentException("Directory cannot be read: " + aDirectory);  } }}