Java - GregorianCalendar

Question

I'm using the GregorianCalendar in Java, and I am wondering how I can use this to check whether or not a date is valid (E.g.: to check if Feb 29th is only in leap year, to check if the date is no sooner than the current data, etc).

I have created a GregorianCalendar object and passed it the values of the data I would like to check as follows:

GregorianCalendar cal1 = new GregorianCalendar(day,month,year);

If the date is valid, I'd like to return true. How could I do this?

Answer 1

Basic Idea: if you try to set the invalid date to Calendar instance, it would make it correct one,

For example if you set 45 as date it would not be the same once you set and retrieve

public boolean isValid(int d, int m, int y){
    //since month is 0 based
    m--;
    //initilizes the calendar instance, by default the current date
    Calendar cal = Calendar.getInstance();
    //resetting the date to the one passed
    cal.set(Calendar.YEAR, y);
    cal.set(Calendar.MONTH, m);
    cal.set(Calendar.DATE, d);

    //now check if it is the same as we set then its valid, not otherwise
    if(cal.get(Calendar.DATE)==d &&cal.get(Calendar.MONTH) ==m && cal.get(Calendar.YEAR) ==y){
      return true;
    }
    //changed so not valid 
    return false;

}

Answer 2

Check that after creation, the day, month and year is still the same as the original values you passed. If the original values are incorrect, the date will get adjusted accordingly. E.g.. if you pass (29, 1, 2011) - note that the month value is 0-based so 1 is for February -, you will get back (1, 3, 2011).

Answer 3

tl;dr

java.time.LocalDate.of( 2018 , 2 , 31 )

➙ catch DateTimeException for invalid day-of-month number.

java.time

The GregorianCalendar class has been supplanted by the ZonedDateTime class as part of java.time built into Java 8 and later. A new method has been added to the old class for conversion.

ZonedDateTime zdt = myGregCal.toZonedDateTime() ;

You want a date-only value, so use LocalDate, without time-of-day and without time zone.

You can extract a LocalDate from a ZonedDateTime time.

LocalDate ld = zdt.toLocalDate() ;

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 ) ;

Valid value

to check if Feb 29th is only in leap year

The documentation for `LocalDate.of( year , month , day ) says:

The day must be valid for the year and month, otherwise an exception will be thrown.

So catch the DateTimeException.

try { 
    LocalDate ld = LocalDate.of( 2018 , 2 , 31 ) ;  // Invalid, February never has 31 days.
    return Boolean.TRUE ;
} catch ( DateTimeException e ) {
    return Boolean.FALSE ;           
}

Leap Year

Yes, LocalDate checks for Leap Year to handle February 29 correctly.

try { 
    LocalDate ld = LocalDate.of( 2018 , 2 , 29 ) ;  // Invalid, as 2018 is a common year.
    return Boolean.TRUE ;
} catch ( DateTimeException e ) {
    return Boolean.FALSE ;    
}

false

…
    LocalDate ld = LocalDate.of( 2020 , 2 , 29 ) ;  // Valid, as 2020 is a leap year.
…

true

Compare dates

to check if the date is no sooner than the current data

I assume you meant "current date".

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 ) ;

Compare with isBefore, isAfter, and isEqual.

boolean b = ld.isBefore( today ) ;

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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.

Using a JDBC driver compliant with JDBC 4.2 or later, you may exchange java.time objects directly with your database. No need for strings nor java.sql.* classes.

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.