String Permutation

Below is my permutation function and I'm looking to make it more elegant and efficient if possible.

The function should return a string which includes all the permutations of the given string (separated by comma).

private static String permutation(String prefix, String str) {
    int n = str.length();
    String toReturn = "";

    if (n == 0) {
        return prefix + ",";
    } else {
        for (int i = 0; i < n; i++) {
            toReturn +=  permutation(prefix + str.charAt(i), str.substring(0, i) + str.substring(i + 1, n));
        }
    }
    return toReturn;
}