fetch the first date of the week

问题

I want to fetch the first date of the week in java using Calendar and Date with the inputs as :: 1. Week number of the month 2. Month 3. year

    Calendar cal = Calendar.getInstance();
    int weekOfMonth = Calendar.WEEK_OF_MONTH;
    System.out.println("weekOfMonth : " + weekOfMonth);

After this i do not know to proceed.

回答1:

There is a method getFirstDayOfWeek() in Calendar class that will return the day in 0 for Sunday, 1 for Monday and so on.The following code snippet is based on this and will give you the first date of the week.

private static Date getFirstDateOfWeek(){
        Calendar cal = Calendar.getInstance(); 
        Date d = Calendar.getInstance().getTime();
        int currDay  =  d.getDay(); // getting the current day
        int startDay =  (currDay - cal.getFirstDayOfWeek()) + 1; // calculate the difference of number days to the current date from the first day of week
        d.setDate(d.getDate() - startDay); // setting the date accordingly.
        return d;
    }

Hope this will help you.

回答2:

int year = 2012;
int month = 3 //for April(they start from 0 = January)
int week = 2;
Calendar calendar = Calendar.getInstance();
calendar.clear();
calendar.set(Calendar.YEAR, year);
calendar.set(Calendar.MONTH, month);
calendar.set(Calendar.YEEK_OF_MONTH, week);

Date date = calendar.getTime();

回答3:

Using java.time

Firstly, you can represent the year and month with the YearMonth class.

YearMonth ym = YearMonth.of( 2017 , Month.FEBRUARY );

As for week-of-month, you must define your terms. Do you mean the week that contains the first of the month? Or do you mean the first week with all seven days being of that month? Or do you mean the first week with a certain day-of-week?

If you want the first of the month, ask the YearMonth.

LocalDate firstOfMonth = ym.atDay( 1 );

To get the start of the week containing that first-of-month date, use a TemporalAdjuster found in the TemporalAdjusters (note plural name) class.

LocalDate previousOrSameMondayOfFirstOfMonth = 
    firstOfMonth.with( TemporalAdjusters.previousOrSame( DayOfWeek.MONDAY ) ) ;

If you mean the week containing the first occurrence of a certain day-of-week, use another TemporalAdjuster.

LocalDate firstMondayOfMonth = 
  firstOfMonth.with( TemporalAdjusters.firstInMonth( DayOfWeek.MONDAY ) );

---

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.

来源：https://stackoverflow.com/questions/10119782/fetch-the-first-date-of-the-week