java中日期格式化的大坑

  @Test
  public void testWeekBasedYear() {
    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
    SimpleDateFormat formatYYYY = new SimpleDateFormat("YYYY/MM/dd");
    System.out.println("2019-12-31 转 YYYY/MM/dd 格式: " + formatYYYY.format(strDate1));
    System.out.println("2020-01-01 转 YYYY/MM/dd 格式: " + formatYYYY.format(strDate2));
    // 小写 YYYY
    SimpleDateFormat formatyyyy = new SimpleDateFormat("yyyy/MM/dd");
    System.out.println("2019-12-31 转 yyyy/MM/dd 格式: " + formatyyyy.format(strDate1));
    System.out.println("2020-01-01 转 yyyy/MM/dd 格式: " + formatyyyy.format(strDate2));
  }

 private static void tryit(int Y, int M, int D, String pat) {
    DateTimeFormatter fmt = DateTimeFormatter.ofPattern(pat);
    LocalDate     dat = LocalDate.of(Y,M,D);
    String      str = fmt.format(dat);
    System.out.printf("Y=%04d M=%02d D=%02d " +
      "formatted with " +
      "\"%s\" -> %s\n",Y,M,D,pat,str);
  }
  public static void main(String[] args){
    tryit(2020,01,20,"MM/DD/YYYY");
    tryit(2020,01,21,"DD/MM/YYYY");
    tryit(2020,01,22,"YYYY-MM-DD");
    tryit(2020,03,17,"MM/DD/YYYY");
    tryit(2020,03,18,"DD/MM/YYYY");
    tryit(2020,03,19,"YYYY-MM-DD");
  }