简介

伴随 lambda表达式、 streams 以及一系列小优化，Java 8 推出了全新的日期时间API。

Java处理日期、日历和时间的不足之处：将 java.util.Date 设定为可变类型，以及 SimpleDateFormat的非线程安全使其应用非常受限。然后就在 java8 上面增加新的特性。

全新API的众多好处之一就是，明确了日期时间概念，例如： 瞬时（instant）、 长短（duration）、 日期、 时间、 时区和周期。

同时继承了Joda库按人类语言和计算机各自解析的时间处理方式。不同于老版本，新API基于ISO标准日历系统，java.time包下的所有类都是不可变类型而且线程安全。

关键类

• Instant：瞬时实例。

• LocalDate：本地日期，不包含具体时间 例如：2014-01-14 可以用来记录生日、纪念日、加盟日等。

• LocalTime：本地时间，不包含日期。

• LocalDateTime：组合了日期和时间，但不包含时差和时区信息。

• ZonedDateTime：最完整的日期时间，包含时区和相对UTC或格林威治的时差。

新API还引入了 ZoneOffSet 和 ZoneId 类，使得解决时区问题更为简便。解析、格式化时间的
DateTimeFormatter\ 类也全部重新设计。

实战

1.获取今天的日期

    @Test
    public void getCurrentDate(){
        LocalDate today = LocalDate.now();
        System.out.println("Java8's Local date : " + today);

        //这个是老版本
        Date date = new Date();
        System.out.println("Old version date : "+date);
    }

    public void getDetailDate(){
        LocalDate today = LocalDate.now();
        int year = today.getYear();
        int month = today.getMonthValue();
        int day = today.getDayOfMonth();

        System.out.printf("Year : %d  \nMonth : %d  \nday : %d ", year, month, day);
    }

    public void handleSpecilDate(){
        LocalDate dateOfBirth = LocalDate.of(2020, 01, 23);
        System.out.println("The specil date is : " + dateOfBirth);
    }

    public void compareDate(){
        LocalDate today = LocalDate.now();
        LocalDate date1 = LocalDate.of(2020, 01, 23);

        if(date1.equals(today)){
            System.out.printf("TODAY %s and DATE1 %s are same date %n", today, date1);
        }else{
            System.out.printf("TODAY %s and DATE1 %s are not same date %n", today, date1);
        }
    }

    public void cycleDate(){
        LocalDate today = LocalDate.now();
        LocalDate dateOfBirth = LocalDate.of(2020, 01, 23);

		//某月某日
        MonthDay birthday = MonthDay.of(dateOfBirth.getMonth(), dateOfBirth.getDayOfMonth());
        MonthDay currentMonthDay = MonthDay.from(today);

        if(currentMonthDay.equals(birthday)){
            System.out.println("Many Many happy returns of the day !!");
        }else{
            System.out.println("Sorry, today is not your birthday");
        }
    }

    public void getCurrentTime(){
        LocalTime time = LocalTime.now();
        System.out.println("local time now : " + time);
    }

    public void plusHours(){
        LocalTime time = LocalTime.now();
        LocalTime newTime = time.plusHours(2); // 增加两小时
        System.out.println("Time after 2 hours : " +  newTime);
		
        newTime = time.plusHours(-2);// 减少两小时
        //newTime = time.minusHours(2);//当然也可以用minusHours方法实现上行代码同样效果
        System.out.println("Time before 2 hours : " +  newTime);
    }

    public void nextWeek(){
        LocalDate today = LocalDate.now();
        LocalDate nextWeek = today.plus(1, ChronoUnit.WEEKS);
        System.out.println("Today is : " + today);
        System.out.println("Date after 1 week : " + nextWeek);
    }

    public void minusDate(){
        LocalDate today = LocalDate.now();
        LocalDate previousYear = today.minus(1, ChronoUnit.YEARS);
        System.out.println("Date before 1 year : " + previousYear);

        LocalDate nextYear = today.plus(1, ChronoUnit.YEARS);
        System.out.println("Date after 1 year : " + nextYear);
    }

    public void clock(){
        // 根据系统时间返回当前时间并设置为UTC。
        Clock clock = Clock.systemUTC();
        System.out.println("Clock : " + clock);

        // 根据系统时钟区域返回时间
        Clock defaultClock = Clock.systemDefaultZone();
        System.out.println("Clock : " + clock);
    }

    public void isBeforeOrIsAfter(){
        LocalDate today = LocalDate.now();

        LocalDate tomorrow = LocalDate.of(2020, 1, 29);
        if(tomorrow.isAfter(today)){
            System.out.println("Tomorrow comes after today");
        }

        LocalDate yesterday = today.minus(1, ChronoUnit.DAYS);

        if(yesterday.isBefore(today)){
            System.out.println("Yesterday is day before today");
        }
    }

    public void getZoneTime(){
        //设置时区
        ZoneId america = ZoneId.of("America/New_York");

        LocalDateTime localtDateAndTime = LocalDateTime.now();

        ZonedDateTime dateAndTimeInNewYork  = ZonedDateTime.of(localtDateAndTime, america );
        System.out.println("现在的日期和时间在特定的时区 : " + dateAndTimeInNewYork);
    }

    public void checkCardExpiry(){
        YearMonth currentYearMonth = YearMonth.now();
        System.out.printf("Days in month year %s: %d%n", currentYearMonth, currentYearMonth.lengthOfMonth());

        YearMonth creditCardExpiry = YearMonth.of(2028, Month.FEBRUARY);
        System.out.printf("Your credit card expires on %s %n", creditCardExpiry);
    }

    public void isLeapYear(){
        LocalDate today = LocalDate.now();
        if(today.isLeapYear()){
            System.out.println("This year is Leap year");
        }else {
            System.out.println("2020 is not a Leap year");
        }
    }

    public void calcDateDays(){
        LocalDate today = LocalDate.now();

        LocalDate java8Release = LocalDate.of(2020, Month.MAY, 14);

        Period periodToNextJavaRelease = Period.between(today, java8Release);

        System.out.println("Months left between today and Java 8 release : "
                + periodToNextJavaRelease.getMonths() );
    }

    public void ZoneOffset(){
        LocalDateTime datetime = LocalDateTime.of(2020, Month.FEBRUARY, 14, 19, 30);
        ZoneOffset offset = ZoneOffset.of("+05:30");
        OffsetDateTime date = OffsetDateTime.of(datetime, offset);
        System.out.println("Date and Time with timezone offset in Java : " + date);
    }

    public void getTimestamp(){
        Instant timestamp = Instant.now();
        System.out.println(""What's the value of this instant : "" + timestamp);
    }

    public void formateDate(){
        String dayAfterTommorrow = "20200123";
        LocalDate formatted = LocalDate.parse(dayAfterTommorrow, DateTimeFormatter.BASIC_ISO_DATE);
        System.out.printf("Date generated from String %s is %s %n", dayAfterTommorrow, formatted);
    }