public static void permute(String s) {
    permute(s, "");
}

private static void permute(String s, String chosen) {
    if (s.length() == 0) {
         // base case: no choices left to be made
         System.out.println(chosen);
     } else {
         // recursive case: choose each possible next letter
         for (int i = 0; i < s.length(); i++) {
             char c = s.charAt(i); // choose
             s = s.substring(0, i) + s.substring(i + 1);
             chosen += c;
             permute(s, chosen); // explore
             s = s.substring(0, i) + c + s.substring(i);
             chosen = chosen.substring(0, chosen.length() - 1); //un-choose
         }                                     
     }
}