I'm not quite sure what field to use when adding more than 30 days to a Java Calendar object. Is there any difference in between Calendar.DAY_OF_MONTH and Calendar.DAY_OF_YEAR?

Example:

GregorianCalendar d = new GregorianCalendar();
d.add(Calendar.DAY_OF_YEAR, 90);

vs

GregorianCalendar d = new GregorianCalendar();
d.add(Calendar.DAY_OF_MONTH, 90);

T开发者_运维问答hanks.

I don't think it makes a difference when you call add. The distinction is important when you call the getters.

Both methods work fine, right? For more than 30 days, as well as negative amounts.

The (admittedly complicated) source for GregorianCalendar#add has this section:

 case DAY_OF_MONTH: // synonym of DATE
 case DAY_OF_YEAR:
 case DAY_OF_WEEK:
    break;

tl;dr

LocalDate.now( ZoneId.of( "America/Montreal" ) )
         .plusDays( 30 )

Details

Much easier now with the modern java.time classes that supplant the old Calendar & Date 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.

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 );

You can add a number of days to that.

LocalDate later = today.plusDays( 30 );

Period

You can represent a span of time with the Period class.

Period thirtyDays = Period.ofDays( 30 );

You can perform date math by calling plus or minus methods.

LocalDate later = today.plus( thirtyDays );

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 and 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 SE 7
  • Much of the java.time functionality is back-ported to Java 6 & 7 in ThreeTen-Backport.
• Android
  • The ThreeTenABP project adapts ThreeTen-Backport (mentioned above) for Android specifically.
  • 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.