YYYYMMdd 和 yyyyMMdd 的区别

来自 https://www.v2ex.com/t/633650

YYYY 是表示：当天所在的周属于的年份，一周从周日开始，周六结束，只要本周跨年，那么这周就算入下一年。

If a week is split at the end of the year then it is assigned to the year in which more that half of the days of that week occur.

u year
y year-of-era
Y week-based-year

   public static void main(String[] args) {  
       DateFormat df = new SimpleDateFormat("yyyyMMdd");  
       System.out.println(df.format(new Date()));  
       DateFormat df1 = new SimpleDateFormat("YYYYMMdd");  
       System.out.println(df1.format(new Date()));  
       Calendar calendar = Calendar.getInstance();  
       // 2019-12-31  
       calendar.set(2019, Calendar.DECEMBER, 31);  
       Date strDate1 = calendar.getTime();  
       // 2020-01-01  
       calendar.set(2020, Calendar.JANUARY, 1);  
       Date strDate2 = calendar.getTime();  
       // 大写 YYYY 
        DateFormat formatUpperCase = new SimpleDateFormat("YYYY/MM/dd");  
       System.out.println("2019-12-31 to YYYY/MM/dd: " \+ formatUpperCase.format(strDate1));  
       System.out.println("2020-01-01 to YYYY/MM/dd: " \+ formatUpperCase.format(strDate2));  
       // 小写 YYYY  
       DateFormat formatLowerCase = new SimpleDateFormat("yyyy/MM/dd");  
       System.out.println("2019-12-31 to yyyy/MM/dd: " \+ formatLowerCase.format(strDate1));  
       System.out.println("2020-01-01 to yyyy/MM/dd: " \+ formatLowerCase.format(strDate2));  
   }