code: 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();
      }
docs: 得到固定长度的随机字符串，字符串由sourceChar中字符混合组成
  
  @param sourceChar 源字符数组
  @param length     长度
  @return <ul>
  <li>若sourceChar为null或长度为0，返回null</li>
  <li>若length小于0，返回null</li>
  </ul>