private void buildList(File[] files, ArrayList<File> mp3s) {
	    for (File file : files) {
	        if (file.isDirectory()) {	      
	            buildList(file.listFiles(), mp3s); // Calls same method again.
	        } else {
	            if(file.getName().endsWith(".mp3") == true){
	            	mp3s.add(file);
	            }
	        }
	    }
	}