Output all permutations of a given string

Permutation.java

import java.util.LinkedHashSet;
import java.util.Set;

public class Permutation {
  public static void main(String[] args) {
    String str = "sonya"; // 5 * 4 * 3 * 2 = 20 * 6 = 120
    Permutation permutation = new Permutation();
    StringBuilder builder = new StringBuilder();
    Set<String> permSet = new LinkedHashSet<>();
    permutation.permute(str, builder, permSet);

    for (String s : permSet) {
      System.out.println(s);
    }
    System.out.println(permSet.size());
  }

  public static void indent(int amount) {
    for (int i = 0; i < amount; i++) {
      System.out.print(' ');
    }
  }

  private void permute(final String str, final StringBuilder builder, final Set<String> permSet) {
//    indent(builder.length());
//    System.out.println("permute(" + str + "," + builder.toString() + ")");
    if (str == null || str.isEmpty()) {
      permSet.add(builder.toString());
    } else {
      int n = str.length();
      for (int i = 0; i < n; i++) {
        // choose 1 letter
        char c = str.charAt(i);
        String substring = str.replaceFirst(Character.toString(c), "").trim();
        builder.append(c);

        // explore
        permute(substring, builder, permSet);

        // un-choose
        builder.deleteCharAt(builder.length() - 1);
      }
    }
  }
}