java 获取验证码优化（使用字符串截取，修改整数运算，提升n倍）

int count = 10000;
      long start = System.currentTimeMillis();
      for (int i = 0; i < count; i++) {
         String code = (Math.random() + "").substring(2, 8);
         if (code.length() != 6) {
            System.out.println("生成失败：" + code);
         }
      }
      long end = System.currentTimeMillis();
      System.out.println("截取耗时：" + (end - start));

      System.out.println("===================================================");
      start = System.currentTimeMillis();
      for (int i = 0; i < count; i++) {
         String code = String.valueOf((int) ((Math.random() * 9 + 1) * Math.pow(10, 5)));
         if (code.length() != 6) {
            System.out.println("生成失败：" + code);
         }
      }
      end = System.currentTimeMillis();
      System.out.println("整数运算,执行时间：" + (end - start));