A simple method to get all the sub-folders within another folder in Java.

FileHandlerGetSubFolders.java

    public static ArrayList<File> getSubFolders(File folder) {
        File[] dirs = {folder};
        ArrayList<File> directories = new ArrayList<>(Arrays.asList(dirs));

        File[] fs = {folder};
        ArrayList<File> folders = new ArrayList<>(Arrays.asList(fs));
        while (folders.size() > 0){
            ArrayList<File> remove = new ArrayList<>();
            ArrayList<File> add = new ArrayList<>();

            for (File f : folders){
                File[] subDirectories = f.listFiles((current, name) -> new File(current, name).isDirectory());
                directories.addAll(Arrays.asList(subDirectories));
                add.addAll(Arrays.asList(subDirectories));
                remove.add(f);
            }

            folders.addAll(add);
            folders.removeAll(remove);
        }

        return directories;
    }