java 常用小知识

1 、 replaceAll的用法
       String aa = "{0}_0{1}_98.ECPAY";
       System.out.println(aa.replaceAll("\\{1\\}", "20170809"));
       打印结果：{0}_020170809_98.ECPAY     //表示转译 将{1} 转换成20170809

2、java获取昨天、今天的时间
      public static void testCalendar() {
        int value=-1;//获取今天相差的天数
        Calendar c = Calendar.getInstance();
        c.add(Calendar.DAY_OF_MONTH, value);
        SimpleDateFormat myFormatter = new SimpleDateFormat("yyyyMMdd");
        String yesterday = myFormatter.format(c.getTime());
        System.out.println(yesterday);
     }