Retrieve current week's Monday's date

问题:

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());

回答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.

回答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.

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.

---

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.

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.

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 (<26), 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.

回答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.

回答4:

As Jon suggested, the calendar.set method works... I've tested it both in the case of a monday in same month and in another month using following snippet :

Calendar c = Calendar.getInstance();
//ensure the method works within current month
c.set(Calendar.DAY_OF_WEEK, Calendar.MONDAY);
System.out.println("Date " + c.getTime());
//go to the 1st week of february, in which monday was in january
c.set(Calendar.DAY_OF_MONTH, 1);
System.out.println("Date " + c.getTime());
//test that setting day_of_week to monday gives a date in january
c.set(Calendar.DAY_OF_WEEK, Calendar.MONDAY);
System.out.println("Date " + c.getTime());
//same for tuesday
c.set(Calendar.DAY_OF_WEEK, Calendar.TUESDAY);
System.out.println("Date " + c.getTime());

The results:

Date Mon Feb 13 10:29:41 CET 2012
Date Wed Feb 01 10:29:41 CET 2012
Date Mon Jan 30 10:29:41 CET 2012
Date Tue Jan 31 10:29:41 CET 2012

回答5:

What about using Joda Time library... Take look at this answer...

回答6:

in case you don't want to use Joda Time you can do like this to find the weeks -> (works on Android)

public static ArrayList<String> getWeeks(int month) {

ArrayList<String> arrayListValues = new ArrayList<>();
SimpleDateFormat sdf = new SimpleDateFormat("MMMM d");

Calendar calendar = Calendar.getInstance();

calendar.set(Calendar.MONTH, month);

int day = calendar.get(Calendar.DAY_OF_WEEK);
while (day != 1) {
  calendar.add(Calendar.DATE, 1);
  day = calendar.get(Calendar.DAY_OF_WEEK);
}

calendar.add(Calendar.DATE, -14);
String y1 = sdf.format(calendar.getTime());
calendar.add(Calendar.DATE, 6);
String y2 = sdf.format(calendar.getTime());
calendar.add(Calendar.DATE, 1);
arrayListValues.add(y1 + " - " + y2);

 y1 = sdf.format(calendar.getTime());
calendar.add(Calendar.DATE, 6);
 y2 = sdf.format(calendar.getTime());
calendar.add(Calendar.DATE, 1);
arrayListValues.add(y1 + " - " + y2);



for (int i = 0; i < 10; i++) {

  y1 = sdf.format(calendar.getTime());
  calendar.add(Calendar.DATE, 6);
  y2 = sdf.format(calendar.getTime());
  calendar.add(Calendar.DATE, 1);
  arrayListValues.add(y1 + " - " + y2);
}

return arrayListValues;
}

回答7:

It won't work if the currents week monday is the month before... For example if today is Friday 1st of June... You should probably rather use the roll method...