String Permutation

import java.util.Arrays;
import java.util.HashSet;
import java.util.List;
import java.util.Set;
import java.util.stream.Collectors;

public class Test {

    public static Set<String> getPermutation(String str) {

        // create a set to avoid duplicate permutation
        Set<String> permutations = new HashSet<>();

        // check if string is null
        if (str == null) {
            return null;
        } else if (str.length() == 0) {
            // terminating condition for recursion
            permutations.add("");
            return permutations;
        }

        // get the first character
        char first = str.charAt(0);

        // get the remaining substring
        String sub = str.substring(1);

        // make recursive call to getPermutation()
        Set<String> words = getPermutation(sub);

        // access each element from words
        for (String strNew : words) {
            for (int i = 0;i<=strNew.length();i++){

                // insert the permutation to the set
                permutations.add(strNew.substring(0, i) + first + strNew.substring(i));
            }
        }
        return permutations;
    }
    public static void main(String[] args) {
        //findAllPossibleGreaterNumber();
        System.out.println(getPermutation("ABC"));
    }

//    private static void findAllPossibleGreaterNumber() {
//        int[] array = {17, 6, 9, 16, 5, 13, 8, 2, 1};
//        List<Integer> list= Arrays.stream(array).boxed().collect(Collectors.toList());
//        for (int i = 0; i <= array.length-1 ; i++) {
//            list.remove(Integer.valueOf(array[i]));
//            if(!list.isEmpty()) {
//                if (array[i] >= list.stream().max(Integer::compareTo).get())
//                    System.out.println(array[i]);
//            }
//        }
//    }

}