每月的一天-Java日历

Question

I want get, how much month have got days. I have got this code:

    Calendar cal = Calendar.getInstance();
    cal.set(2013, 1, 1);
    System.out.println(cal.getActualMaximum(cal.DAY_OF_MONTH));

Januar of year 2013 (and other else yaers :)) have got 31 days, but, I'm still getting 28, do you know, where I have got mistake? Thank you.

Answer 1

The first month of the year in the Gregorian and Julian calendars is JANUARY which is 0; the last depends on the number of months in a year. You are getting the max days for February.

Try cal.set(2013, 0, 1);

Answer 2

@sunrize920 is spot on. To avoid making mistakes, I find it easier to use the Calender deifned values as much as possible. So in your desired case that would be:

cal.set(2013, Calendar.JANUARY, 1);

Much easier, and saves remembering whether Calendar counts months from 0 or 1 ...

Answer 3

If you want to get number of days for January of 2013

try

cal.set(2013,0,1);

Answer 4

tl;dr

YearMonth.from(                               // Represent the month as a whole with `YearMonth` class.
    LocalDate.of( 2013 , Month.JANUARY , 1 )  // A date-only value, without time-of-day and without time zone.
)
.lengthOfMonth()                              // As for the month’s length. Returns an `int`.

31

java.time

The modern approach uses the java.time classes that supplanted the troublesome old date-time classes such as Date & Calendar .

LocalDate

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 ) ;

If you want to use the JVM's current default time zone, ask for it and pass as an argument. If omitted, the JVM's current default is applied implicitly. Better to be explicit, as the default may be changed at any moment during runtime by any code in any thread of any app within the JVM.

ZoneId z = ZoneId.systemDefault() ;  // Get JVM’s current default time zone.

Or specify a date. You may set the month by a number, with sane numbering 1-12 for January-December.

LocalDate ld = LocalDate.of( 1986 , 2 , 23 ) ;  // Years use sane direct numbering (1986 means year 1986). Months use sane numbering, 1-12 for January-December.

Or, better, use the Month enum objects pre-defined, one for each month of the year. Tip: Use these Month objects throughout your codebase rather than a mere integer number to make your code more self-documenting, ensure valid values, and provide type-safety .

LocalDate ld = LocalDate.of( 1986 , Month.FEBRUARY , 23 ) ;

YearMonth

For that LocalDate , get its entire month, represented by the YearMonth class.

YearMonth ym = YearMonth.from( ld ) ;

Interrogate that YearMonth object for its length, the number of days in the month.

int lengthOfMonth = ym.lengthOfMonth() ;

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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 .

You may exchange java.time objects directly with your database. Use a JDBC driver compliant with JDBC 4.2 or later. No need for strings, no need for java.sql.* classes.

Where to obtain the java.time classes?

• Java SE 8 , Java SE 9 , Java SE 10 , 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 (<26), 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 .