code: public static void populateFilesList(File dir, List<File> filesListInDir) {
  		File[] files = dir.listFiles();
  		for(File file : files){
  			if(file.isFile()){
  				filesListInDir.add(file);
  			}else{
  				populateFilesList(file, filesListInDir);
  			}
  		}
  	}
docs: Generate the list of files in the directory and all of its sub-directories (recursive)
  
  @param dir - The directory
  @param filesListInDir - List to store the files