Retrieve current week's Monday's date

Question

We have a utility that will run any day between Monday - Friday. It will update some number of files inside a Content Management Tool. The last modified date associated with that file should be, that week's monday's date. I wrote the following program to retrieve current week's monday's date. But I am still not sure whether this would work for all scenarios. Has anyone got a better solution ?

Calendar c = Calendar.getInstance();
c.setTime(new Date());
System.out.println(c.get(Calendar.DAY_OF_MONTH));
System.out.println(c.get(Calendar.DAY_OF_WEEK));
int mondayNo = c.get(Calendar.DAY_OF_MONTH)-c.get(Calendar.DAY_OF_WEEK)+2;
c.set(Calendar.DAY_OF_MONTH,mondayNo);
System.out.println("Date "+c.getTime());

Answer 1

I would strongly recommend using Joda Time instead (for all your date/time work, not just this):

// TODO: Consider time zones, calendars etc
LocalDate now = new LocalDate();
LocalDate monday = now.withDayOfWeek(DateTimeConstants.MONDAY);
System.out.println(monday);

Note that as you've used Monday here, which is the first day of the week in Joda Time, this will always return an earlier day (or the same day). If you chosen Wednesday (for example), then it would advance to Wednesday from Monday or Tuesday. You can always add or subtract a week if you need "the next Wednesday" or "the previous Wednesday".

EDIT: If you really want to use java.util.Date/Calendar, you can use:

Calendar c = Calendar.getInstance();
c.set(Calendar.DAY_OF_WEEK, Calendar.MONDAY);
System.out.println("Date " + c.getTime());

You can use Calendar.setFirstDayOfWeek to indicate whether a week is Monday-Sunday or Sunday-Saturday; I believe setting the day of the week will stay within the current week - but test it.

Answer 2

tl;dr

LocalDate previousMonday = 
    LocalDate.now( ZoneId.of( "America/Montreal" ) )
             .with( TemporalAdjusters.previous( DayOfWeek.MONDAY ) );

java.time

Both the java.util.Calendar class and the Joda-Time library have been supplanted by the java.time framework built into Java 8 and later. See Oracle Tutorial. Much of the java.time functionality has been back-ported to Java 6 & 7 in ThreeTen-Backport and further adapted to Android in ThreeTenABP. With modern Android tooling and its "API desugaring", you need not add any library.

The java.time.LocalDate class represents a date-only value without time-of-day and without time zone.

Determining today's date requires a time zone, a ZoneId.

ZoneId zoneId = ZoneId.of( "America/Montreal" );
LocalDate today = LocalDate.now( zoneId );

The TemporalAdjuster interface (see Tutorial) is a powerful but simple way to manipulate date-time values. The TemporalAdjusters class (note the plural) implements some very useful adjustments. Here we use previous( DayOfWeek).

The handy DayOfWeek enum makes it easy to specify a day-of-week.

LocalDate previousMonday = today.with( TemporalAdjusters.previous( DayOfWeek.MONDAY ) );

If today is Monday, and you want to use today rather than a week ago, call previousOrSame.

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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.

To learn more, see the Oracle Tutorial. And search Stack Overflow for many examples and explanations. Specification is JSR 310.

The Joda-Time project, now in maintenance mode, advises migration to the java.time classes.

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. Hibernate 5 & JPA 2.2 support java.time.

Where to obtain the java.time classes?

• Java SE 8, Java SE 9, Java SE 10, Java SE 11, and later - Part of the standard Java API with a bundled implementation.
  • Java 9 brought some minor features and fixes.
• Java SE 6 and Java SE 7
  • Most of the java.time functionality is back-ported to Java 6 & 7 in ThreeTen-Backport.
• Android
  • Later versions of Android (26+) bundle implementations of the java.time classes.
  • For earlier Android (<26), the process of API desugaring brings a subset of the java.time functionality not originally built into Android.
    • If the desugaring does not offer what you need, the ThreeTenABP project adapts ThreeTen-Backport (mentioned above) to Android. 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 3

The following will work, including wrapping months:

Calendar c = Calendar.getInstance();
c.setFirstDayOfWeek(Calendar.MONDAY);
c.setTime(new Date());
int today = c.get(Calendar.DAY_OF_WEEK);
c.add(Calendar.DAY_OF_WEEK, -today+Calendar.MONDAY);
System.out.println("Date "+c.getTime());

If, however, you edit your application on a Sunday (eg. Sunday 12 Feb), the date will be for the following Monday. Based on your requirements (the app will only run Monday thru Friday), this should not pose a problem.