android 创建文件夹

mkdir() 只能创建一层目录.  

mkdirs()，才能创建多层目录

private final String APP_FOLDER = "/sdcard/DownDemo/"; // sd卡应用目录
File folder = null;
folder = new File(Environment.getExternalStorageDirectory()+ File.separator + APP_FOLDER );
                if (!folder.exists()) {
                    folder.mkdirs();
                }
private final String APP_FOLDER = "DownDemo"; // sd卡应用目录
folder = new File(Environment.getExternalStorageDirectory()+ File.separator + APP_FOLDER );
                if (!folder.exists()) {
                    folder.mkdir();
                }