Modify the week in a Calendar

Question

Hello i'm using a Calendar in Java

What I want is to change my Calendar value, and have the following / previous week.

I tried

Calendar cal = Calendar.getInstance();
cal.set(Calendar.DATE, cal.DATE-7);

But it acts weird

Answer 1

Have you tried (copied/pasted from javadoc):

cal.add(Calendar.DAY_OF_MONTH, -7);

Answer 2

You should use something like the following instead:

c.add(Calendar.WEEK_OF_YEAR, -1);

Your code is completely wrong. cal.DATE-7 does not mean "7 days before today". Calendar.DATE is just a constant that specifies the field type for Calendar.

Answer 3

tl;dr

Next week.

LocalDate.now( ZoneId.of( "America/Montreal" ) )
         .plusWeeks( 1L )

Last week.

LocalDate.now( ZoneId.of( "America/Montreal" ) )
         .minusWeeks( 1L )

Using java.time

The modern way is with the java.time classes that supplant the troublesome old legacy classes such as Calendar & Date.

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 );

Add/subtract weeks

Adding or subtracting weeks with a LocalDate is easy. Call the plusWeeks or minusWeeks methods.

LocalDate nextWeek = today.plusWeeks( 1L );
LocalDate lastWeek = today.minusWeeks( 1L );

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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 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.

Answer 4

You'll have to do something like this:

public static void main(String[] args) {

    Calendar cal = Calendar.getInstance();
    System.out.println(cal.getTime());
    cal.set(Calendar.WEEK_OF_YEAR, cal.get(Calendar.WEEK_OF_YEAR)-1);
    System.out.println(cal.getTime());
} 

Hope this helped, have Fun!