docs: public static String getRandom(char[] sourceChar, int length) {
          if (sourceChar == null || sourceChar.length == 0 || length < 0) {
              return null;
          }
          StringBuilder str = new StringBuilder(length);
          Random random = new Random();
          for (int i = 0; i < length; i++) {
              str.append(sourceChar[random.nextInt(sourceChar.length)]);
          }
          return str.toString();
      }