code: public static String random(final int count, final char... chars) {
          if (chars == null) {
              return random(count, 0, 0, false, false, null, RANDOM);
          }
          return random(count, 0, chars.length, false, false, chars, RANDOM);
      }
docs: <p>Creates a random string whose length is the number of characters
  specified.</p>
  
  <p>Characters will be chosen from the set of characters specified.</p>
  
  @param count  the length of random string to create
  @param chars  the character array containing the set of characters to use,
  may be null
  @return the random string
  @throws IllegalArgumentException if {@code count} &lt; 0.