java8 LocalDateTime

java8使用了LocalDateTime和DateTimeFormatter。比之前的Date和Carlendar有所改進。
DateTimeFormatter是線程安全的。DateTimeFormatter中很多屬性使用了final修飾。

LocalDate: 只能設置僅含年月日的格式，表示沒有時區的日期, LocalDate是不可變並且線程安全的
LocalTime: 只能設置僅含時分秒的格式，表示沒有時區的時間, LocalTime是不可變並且線程安全的
LocalDateTime: 可以設置含年月日時分秒的格式 ， 表示沒有時區的日期時間, LocalDateTime是不可變並且線程安全的

ZoneId: 時區ID，用來確定Instant和LocalDateTime互相轉換的規則
Instant: 用來表示時間線上的一個點（瞬時）
Clock: 用於訪問當前時刻、日期、時間，用到時區
Duration: 用秒和納秒表示時間的數量（長短），用於計算兩個日期的“時間”間隔
Period: 用於計算兩個“日期”間隔

LocalDateTime

構建LocalDateTime對象：

	public  void generateLocalDate() {
		LocalDate date = LocalDate.of(2000, 1, 8);
		LocalDateTime dateTime = LocalDateTime.of(2020, 1, 8, 15, 16, 17);
		System.out.println("date：" + date);
		System.out.println("dateTime:" + dateTime);
	}

獲取LocalDateTime的年月日：

	public  void getLocalDateTimeMonth() {
		LocalDateTime dateTime = LocalDateTime.of(2020, 1, 8, 15, 16, 17);
		int year=dateTime.getYear();
		int month=dateTime.getMonth().getValue();
		int day=dateTime.getDayOfMonth();
		System.out.println("dateTime對應的年份為："+year);
		System.out.println("dateTime對應的月份為："+month);
		System.out.println("dateTime對應的天數為："+day);
	}

字符串String轉LocalDateTime：

	public  void stringToTime() {
		DateTimeFormatter formatter = DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss");
		String datetimeText = "2020-04-01 23:59:59";
		LocalDateTime localDateTime = LocalDateTime.parse(datetimeText, formatter);
		// LocalDateTime的toString()方法，在日期和時間中間加入了"T"字符串
		System.out.println(localDateTime);
	}

LocalDateTime 轉字符串：

	public  void localDateTimeToString() {
		DateTimeFormatter formatter = DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss");
		LocalDateTime dateTime = LocalDateTime.of(2020, 1, 8, 15, 16, 17);
		String dateTimeStr = dateTime.format(formatter);
		System.out.println("dateTimeStr為：" + dateTime);
	}

Date轉Instant：

        Instant instant= date.toInstant();

Instant轉Date：

        Date date = Date.from(instant);

Date轉LocalDateTime ：

public static void dateToLocalDateTime(Date date) {
        Instant dateInstant = date.toInstant();
        LocalDateTime localDateTime = LocalDateTime.ofInstant(dateInstant, ZoneId.systemDefault());
    }

LocalDateTime 獲取LocalDate：

LocalDate localDate = localDateTime.toLocalDate();

LocalDateTime 獲取LocalTime：

LocalTime localTime = localDateTime.toLocalTime();

LocalDate

Date轉LocalDate：

    public void dateToLocalDate(Date date) {
        Instant dateInstant = date.toInstant();
        LocalDate localDate = LocalDateTime.ofInstant(dateInstant, ZoneId.systemDefault()).toLocalDate();
    }

localDate轉String ：

public static String localDateToString(Date date) {
        Instant dateInstant = date.toInstant();
        LocalDate localDate = LocalDateTime.ofInstant(dateInstant, ZoneId.systemDefault()).toLocalDate();
        // localDate轉String 
        DateTimeFormatter formatter = DateTimeFormatter.ofPattern("yyyy-MM-dd");
        String localDateStr = localDate.format(formatter);
        System.out.println(localDateStr);
        return localDateStr;
    }

String轉LocalDate：

public static void stringToLocalDate() {
        DateTimeFormatter formatter = DateTimeFormatter.ofPattern("yyyy-MM-dd");
        String datetimeText = "2020-04-01";
        LocalDate localDate = LocalDate.parse(datetimeText, formatter);
        System.out.println(localDate);
    }

LocalDate獲取年月日:

    int year = localDate.getYear();
    int monthValue = localDate.getMonthValue();
    int dayOfMonth = localDate.getDayOfMonth();

LocalDate修改年月日：

    public static void changeYearMonthDay() {
        LocalDate localDate = LocalDate.now();
        //修改年份
        LocalDate withYearResult = localDate.withYear(2020);
        //修改月份
        LocalDate withMonthResult = localDate.withMonth(5);
        //修改日
        LocalDate withDayResult = localDate.withDayOfMonth(20);
        System.out.println( withYearResult + "，" + withMonthResult+ "，"+ withDayResult);
    }

LocalDate比較日期大小compareTo：

compareTo()方法
在兩邊相等（=）的情況下返回 0
在左邊大於（>）右邊時返回 1　
在左邊小於（<）右邊時返回 -1

      public static void localDateCompareTo() {
        DateTimeFormatter formatter = DateTimeFormatter.ofPattern("yyyy-MM-dd");
        String datetimeText = "2020-04-01";
        LocalDate localDate = LocalDate.parse(datetimeText, formatter);

        LocalDate nowDate = LocalDate.now();
        System.out.println(localDate);
        System.out.println("nowDate:"+nowDate);

        if (nowDate.compareTo(localDate) < 0) {
            System.out.println("nowDate小於"+ datetimeText);
        } else {
            System.out.println("nowDate大於等於"+ datetimeText);
        }
    }

LocalDate比較日期的前后：

after 只在大於（>）情況下才為true　(相等時不會）
before 只在小於（<）情況下才為true　(相等時不會）

public static void isBeforeAfter() {
        DateTimeFormatter formatter = DateTimeFormatter.ofPattern("yyyy-MM-dd");
        String datetimeText = "2020-04-01";
        LocalDate localDate = LocalDate.parse(datetimeText, formatter);

        LocalDate nowDate = LocalDate.now();
        System.out.println(localDate);
        System.out.println("nowDate:"+nowDate);

        if (nowDate.isBefore(localDate)) {
            System.out.println(nowDate+ "在"+ datetimeText +"之前");
        } else {
            System.out.println(nowDate+ "在"+ datetimeText +"之后");
        }
    }

獲取日期差距：

	public  void monthDiff() {
		LocalDate date1 = LocalDate.of(2019, 1, 15);
		LocalDate date2 = LocalDate.of(2019, 3, 10);
		Period period = Period.between(date1, date2);
		System.out.println("相差的年數為" + period.getYears() + "，相差的月數為：" + period.getMonths() +
				"，相差的天數為：" + period.getDays());
	}

獲取日期時間間隔：

public static void durationDateTime() {
        LocalDateTime date3 = LocalDateTime.now();
        LocalDateTime date4 = LocalDateTime.of(2018, 1, 13, 22, 30, 10);
        Duration duration = Duration.between(date3, date4);
        System.out.println(date3 + " 與 " + date4 + " 間隔  " + "\n"
                + " 天 :" + duration.toDays() + "\n"
                + " 時 :" + duration.toHours() + "\n"
                + " 分 :" + duration.toMinutes() + "\n"
                + " 毫秒 :" + duration.toMillis() + "\n"
                + " 納秒 :" + duration.toNanos() + "\n"
        );
    }

日期時間計算：

	public void addMonthDay() {
		LocalDateTime dt = LocalDateTime.of(2020, 1, 26, 20, 30, 59);
		System.out.println(dt);
		// 加5天減3小時:
		LocalDateTime dt2 = dt.plusDays(5).minusHours(3);
		System.out.println(dt2); // 2019-10-31T17:30:59
		// 減1月:
		LocalDateTime dt3 = dt2.minusMonths(1);
		System.out.println(dt3); // 2019-09-30T17:30:59
	}

LocalTime

LocalTime獲取時分秒：

        //獲取小時
        int hour = localTime.getHour();
        int hour1 = localTime.get(ChronoField.HOUR_OF_DAY);
        //獲取分
        int minute = localTime.getMinute();
        int minute1 = localTime.get(ChronoField.MINUTE_OF_HOUR);
        //獲取秒
        int second = localTime.getMinute();
        int second1 = localTime.get(ChronoField.SECOND_OF_MINUTE);

參考資料：

https://www.liaoxuefeng.com/wiki/1252599548343744/1303871087444002
https://blog.csdn.net/weixin_38405253/article/details/100765007
https://www.jianshu.com/p/048ee8580639