how to create a folder in android External Storage Directory?

Do it like this :

String folder_main = "NewFolder";

File f = new File(Environment.getExternalStorageDirectory(), folder_main);
if (!f.exists()) {
    f.mkdirs();
}

If you wanna create another folder into that :

File f1 = new File(Environment.getExternalStorageDirectory() + "https://stackoverflow.com/" + folder_main, "product1");
if (!f1.exists()) {
    f1.mkdirs();
}