Java: A way to arrange and combine strings

For example, enter "abc" to print out all possible combinations and eliminate duplicate values.

The so-called permutations and combinations are as follows:

Permutation and combination, string :abc
bca
acb
abc
cba
bac
cab

Number of permutations and combinations: 6

Implementation code (combined with Java8 lambda expression implementation)

import org.junit.Test;

import java.util.ArrayList;
import java.util.HashSet;
import java.util.List;

public class test2 {

  @Test
  public void test3() {


    String input="abc";
    //1. Start to arrange 
    List<String> sortResult = sort(input);
    System.out.println(" Permutation and combination , string :"+input);
    //2. Eliminate duplicate columns 
    HashSet h = new HashSet(sortResult);
    sortResult.clear();
    sortResult.addAll(h);
    //3. A printout 
    sortResult.forEach(e -> System.out.println(e));
    //4. Print the number of 
    System.out.println(" Number of permutations and combinations: " + sortResult.size());

  }

  private List<String> sort(String input) {
    List<String> sortList = new ArrayList();
    if (input == null || "".equals(input)) {
      System.out.println(" Tip: You entered a null character , Please enter valid values! ");
      return new ArrayList();
    }
    char leftChar = input.charAt(0);
    if (input.length() > 1) {
      String rightString = input.substring(1, input.length());
      List<String> rightStringSortedList = sort(rightString);
      rightStringSortedList.forEach((e) -> {
        for (int i = 0; i < e.length() + 1; i++) {
          sortList.add(new StringBuffer(e).insert(i, leftChar).toString());
        }
      });
    } else {
      sortList.add(String.valueOf(leftChar));
    }
    return sortList;
  }
}

If there is a more concise code implementation, please do not be stingy, post and share.