java的时间转轮论

时间格式化
   //时间格式化
   SimpleDateFormat oldFormatter = new SimpleDateFormat("yyyy/MM/dd");
   Date date1 = new Date();
   System.out.println(oldFormatter.format(date1));

   // Java 8
   DateTimeFormatter newFormatter = DateTimeFormatter.ofPattern("yyyy/MM/dd");
   LocalDate date2 = LocalDate.now();
   System.out.println(date2.format(newFormatter));

如何取得年月日、小时分钟秒？如何取得从1970年1月1日0时0分0秒到现在的毫秒数？
   Calendar cal = Calendar.getInstance();
   //年
   System.out.println(cal.get(Calendar.YEAR));
   //月
   System.out.println(cal.get(Calendar.MONTH)); // 0 - 11
   //日
   System.out.println(cal.get(Calendar.DATE));
   //时
   System.out.println(cal.get(Calendar.HOUR_OF_DAY));
   //分
   System.out.println(cal.get(Calendar.MINUTE));
   //秒
   System.out.println(cal.get(Calendar.SECOND));
   /*Java 8*/
   LocalDateTime dt = LocalDateTime.now();
   System.out.println(dt.getYear());
   System.out.println(dt.getMonthValue()); // 1 - 12
   System.out.println(dt.getDayOfMonth());
   System.out.println(dt.getHour());
   System.out.println(dt.getMinute());
   System.out.println(dt.getSecond());

以下方法都能获得毫秒数
   //以下方法都能获得毫秒数
   Calendar.getInstance().getTimeInMillis();
   System.currentTimeMillis();
   Clock.systemDefaultZone().millis(); // Java 8

如何取得某月的最后一天？
   Calendar time = Calendar.getInstance();
   time.getActualMaximum(Calendar.DAY_OF_MONTH);
   System.out.println(time.getActualMaximum(Calendar.DAY_OF_MONTH));

打印昨天的当前时刻?
   Calendar cal = Calendar.getInstance();
   cal.add(Calendar.DATE, -1);
   System.out.println(cal.getTime());
   //Java8
   LocalDateTime today = LocalDateTime.now();
   LocalDateTime yesterday = today.minusDays(1);
   System.out.println(yesterday);

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