Print up down stars using recursion

Write a method getUpDownStars() to print an ascending and descending sequence of integers delimited by a '*'.

getUpDownStars(2, 5) return 2*3*4*5*4*3*2

getUpDownStars(1,3) return 1*2*3*2*1
getUpDownStars(1,1) return 1*1

public class PrintStars1 {

 public static String getUpDownStars(int k, int n) {
  if (k < 0 || n < 0 || k > n)
   throw new IllegalArgumentException("Illegal Argument Exception");

  if (k == n)
   return k + "*" + k;

  return getStars(k, n);
 }

 private static String getStars(int k, int n) {
  if (k == n)
   return k + "";
  String str = k + "*" + getStars(k + 1, n);
  str = str + "*" + (k);
  return str;
 }

}

Following is the junit test case for above application.

import static org.junit.Assert.assertEquals;

import org.junit.Test;

public class PrintStars1Test {

 @Test
 public void test1() {
  assertEquals(PrintStars1.getUpDownStars(2, 2), "2*2");
  assertEquals(PrintStars1.getUpDownStars(2, 3), "2*3*2");
  assertEquals(PrintStars1.getUpDownStars(2, 4), "2*3*4*3*2");
  assertEquals(PrintStars1.getUpDownStars(2, 5), "2*3*4*5*4*3*2");
 }

 @Test(expected = IllegalArgumentException.class)
 public void test2() {
  PrintStars1.getUpDownStars(2, 1);
 }
}