code: private void listFiles(File dir, List<File> results) {
          File[] found = dir.listFiles();
          if ( found != null ) {
              for ( File f : found ) {
                  if (f.isDirectory()) {
                  	//do nothing
                  } else {
                      results.add(f);
                  }
              }
          }
      }
docs: Lists files in the directory given and adds them to the result list
  passed in, optionally adding subdirectories recursively.