1

I wrote simple java program in which I get day of month, days in month and month

see below code :

//Calendar calendar = Calendar.getInstance();
Calendar calendar = new GregorianCalendar();
log.info("day of month: "+calendar.get(Calendar.DAY_OF_MONTH));
calendar.set(Calendar.YEAR, Calendar.MONTH, Calendar.DAY_OF_MONTH);
log.info("days in month: "+calendar.getActualMaximum(Calendar.DAY_OF_MONTH));
log.info("month: "+calendar.get(Calendar.MONTH));

Running above code I get this output: 

day of month: 7
days in month: 31
month: 2

But when I put below statement 

log.info("month: "+calendar.get(Calendar.MONTH));

before

log.info("day of month: "+calendar.get(Calendar.DAY_OF_MONTH));

I get this output: (which is what I want) 

day of month: 7
days in month: 31
month: 5

Can any body help me understand why I get month: 2 ?

mort
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Mohsin AR
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  • This isn't clear. If you swap the lines around, your output should be in a different order. Please post the *real* output. – Oliver Charlesworth Jun 07 '14 at 19:04
  • FYI, the troublesome old date-time classes such as [`java.util.Date`](https://docs.oracle.com/javase/9/docs/api/java/util/Date.html), [`java.util.Calendar`](https://docs.oracle.com/javase/9/docs/api/java/util/Calendar.html), and `java.text.SimpleDateFormat` are now [legacy](https://en.wikipedia.org/wiki/Legacy_system), supplanted by the [java.time](https://docs.oracle.com/javase/9/docs/api/java/time/package-summary.html) classes built into Java 8 & Java 9. See [Tutorial by Oracle](https://docs.oracle.com/javase/tutorial/datetime/TOC.html). – Basil Bourque Feb 09 '18 at 01:22

3 Answers3

2

Youre setting the Calendar field to Calendar.MONTH (value 2) here

calendar.set(Calendar.YEAR, Calendar.MONTH, Calendar.DAY_OF_MONTH);
                               ^

You can do this

calendar.set(2014, Calendar.JUNE, 1);

although the Month 5 is June (since month field starts from 0 for Calendar) which only has 30 days

Reimeus
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  • i want to use system year, month and day that's why i put `Calendar.MONTH` which might be `June` or `5` – Mohsin AR Jun 07 '14 at 19:17
  • The field you are using are meant as field numbers rather than actual values. If you want the system year just leave the value in the `Calendar` _as is_ and just [`set`](http://docs.oracle.com/javase/7/docs/api/java/util/Calendar.html#set(int,%20int)) the day & month – Reimeus Jun 07 '14 at 19:30
1

Look at the source code of Calendar.java of JDK.

  public final static int MONTH = 2;

Here, Calendar.MONTH = 2, Calendar.YEAR=1 and Calendar.DAY_OF_MONTH = 5. You set these constant value to calender using set method like.

calendar.set(Calendar.YEAR, Calendar.MONTH, Calendar.DAY_OF_MONTH);
                 |              |              |
                 1              2              5
Masudul
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0

tl;dr

LocalDate.now()
    .getDayOfMonth()

…and…

YearMonth.from(
    LocalDate.now()
).lengthOfMonth()  // .getMonthValue()  .getYear()

java.time

The modern approach uses the industry-leading java.time classes.

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.

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 ) ;

Parts

Interrogate for the parts as needed.

int dayOfMonth = ld.getDayOfMonth() ;
int month = ld.getMonthValue() ;
int year = ld.getYear() ;

YearMonth

To work with the month as a whole, use YearMonth class.

YearMonth ym = YearMonth.from( ld ) ;

Ask for length of month.

int lengthOfMonth = ym.lengthOfMonth() ;

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?

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.

Basil Bourque
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