Java中的日期算术，从今天起增加3个月

Question

My program is about generating (producing) a Kurosawa and making the customers produce it.

Every time we generate a Kurosawa, we have to print its id, its production date and expiration date, which is 3 months from the production date.

My problem is: How can I calculate the date after 3 months?

Answer 1

Use the built-in Java Calendar API .

Calendar c = Calendar.getInstance();
c.add(Calendar.MONTH, 3);

Refer to the API for exactly how to print out the date, in the format you are looking for.

Answer 2

You could also use the much more powerful and easier to use Joda Time Library :

DateMidnight productionDate = new DateMidnight();
DateMidnight expirationDate = productionDate.plusMonths(3);
System.out.println(expirationDate.toString("dd.MM.yyyy"));

Joda Time has many advantages over the built-in Java Calendar API.

Answer 3

tl;dr

LocalDate.now()            // Determine today’s date.
         .plusMonths( 3 )  // Add three months.

java.time

The modern approach uses java.time classes.

The LocalDate class represents a date-only value without time-of-day and without time zone.

A time zone is crucial in determining a date. For any given moment, the date varies around the globe by zone. For example, a few minutes after midnight in Paris France is a new day while still “yesterday” in Montréal Québec .

If no time zone is specified, the JVM implicitly applies its current default time zone. That default may change at any moment, so your results may vary. Better to specify your desired/expected time zone explicitly as an argument.

Specify a proper time zone name in the format of continent/region , such as America/Montreal , Africa/Casablanca , or Pacific/Auckland . Never use the 3-4 letter abbreviation such as EST or IST as they are not true time zones, not standardized, and not even unique(!).

ZoneId z = ZoneId.of( "America/Montreal" );
LocalDate today = LocalDate.now( z );

Add three months, letting java.time do the math.

LocalDate threeMonthsLater = today.plusMonths( 3 ) ;

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About java.time

The java.time framework is built into Java 8 and later. These classes supplant the troublesome old legacy date-time classes such as java.util.Date , Calendar , & SimpleDateFormat .

The Joda-Time project, now in maintenance mode , advises migration to the java.time classes.

To learn more, see the Oracle Tutorial . And search Stack Overflow for many examples and explanations. Specification is JSR 310 .

Where to obtain the java.time classes?

• Java SE 8 , Java SE 9 , and later
  • Built-in.
  • Part of the standard Java API with a bundled implementation.
  • Java 9 adds some minor features and fixes.
• Java SE 6 and Java SE 7
  • Much of the java.time functionality is back-ported to Java 6 & 7 in ThreeTen-Backport .
• Android
  • Later versions of Android bundle implementations of the java.time classes.
  • For earlier Android, the ThreeTenABP project adapts ThreeTen-Backport (mentioned above). See How to use ThreeTenABP… .

The ThreeTen-Extra project extends java.time with additional classes. This project is a proving ground for possible future additions to java.time. You may find some useful classes here such as Interval , YearWeek , YearQuarter , and more .

Answer 4

我相信Java Calendar Library应该可以帮到你。

Answer 5

如果您需要使用日期算术，JODA可以更好地工作，因为日历喜欢时间戳。

Answer 6

/ The java class Calendar is abstract. So you need to use the GregorianCalendar class. /

java.util.GregorianCalendar gC = new java.util.GregorianCalendar();

java.util.Date yourDate = new java.util.Date();
gC.setTime(yourDate);

/ Add 3 months /

gC.add(java.util.Calendar.MONTH_OF_YEAR, 3);

/ to go back 1 week /

gC.add(java.util.Calendar.WEEK_OF_YEAR, -1);