calendar methods

Question

apart from the inbuilt functions, can we use any simple formulas to calculate the start day of the month given month and year as inputs??

Answer 1

Yes - use the Calendar object:

Calendar cal = Calendar.getInstance();
cal.set(Calendar.YEAR, 2009)
cal.set(Calendar.MONTH, Calendar.JUNE) //0-based so that Calendar.JANUARY is 0
cal.set(Calendar.DAY_OF_MONTH, 1)

Then:

cal.get(Calendar.DAY_OF_WEEK); //is Calendar.MONDAY

So you could easily compose a method for this:

/** takes a 1-based month so that Jjanuary is 1 */
public int getDayAtStart(int year, int month) {
    Calendar cal = Calendar.getInstance();
    cal.set(Calendar.YEAR, year)
    cal.set(Calendar.MONTH, month - 1);
    cal.set(Calendar.DAY_OF_MONTH, 1);
    return cal.get(Calendar.DAY_OF_WEEK); 
}

Answer 2

As ever, I'd recommend using Joda Time. I dare say java.util.Calendar will work for this, but I prefer to use Joda Time for all date and time calculations wherever possible. The built-in APIs are horrible. Joda makes this very easy indeed. (Unless specified, LocalDate assumes an ISOChronology - I expect that's what you want, but you can specify a different one if you need to.)

import org.joda.time.*;

public class Test
{
    // Just for the sake of a simple test program!
    public static void main(String[] args) throws Exception
    {
        // Prints 1 (Monday)
        System.out.println(getFirstDayOfMonth(2009, 6));
    }

    /** Returns 1 (Monday) to 7 (Sunday) */
    private static int getFirstDayOfMonth(int year, int month)
    {
        LocalDate date = new LocalDate(year, month, 1);
        return date.getDayOfWeek();
    }
}

Or if you just need to do this in one place, you can mash it into a single line if you want:

int day = new LocalDate(year, month, 1).getDayOfWeek();

It's hard to see how it could be much simpler than that :)

Answer 3

java.time

The java.util Date-Time API and their formatting API, SimpleDateFormat are outdated and error-prone. It is recommended to stop using them completely and switch to the modern Date-Time API^*.

Also, quoted below is a notice at the Home Page of Joda-Time:

Note that from Java SE 8 onwards, users are asked to migrate to java.time (JSR-310) - a core part of the JDK which replaces this project.

Solution using java.time, the modern API:

import static java.lang.System.out;

import java.time.DayOfWeek;
import java.time.YearMonth;
import java.time.format.DateTimeFormatter;
import java.time.format.TextStyle;
import java.util.Locale;

public class Main {
    public static void main(String[] args) {
        DayOfWeek dow = getFirstDayOfMonth(2021, 5);
        out.println(dow); // SATURDAY

        // As weekday number
        out.println(dow.getValue()); // 6

        // Some predefined styles
        out.println(dow.getDisplayName(TextStyle.FULL, Locale.ENGLISH)); // Saturday
        out.println(dow.getDisplayName(TextStyle.SHORT, Locale.ENGLISH)); // Sat
        out.println(dow.getDisplayName(TextStyle.NARROW, Locale.ENGLISH)); // S

        // Formatting using DateTimeFormatter
        out.println(DateTimeFormatter.ofPattern("EEEE", Locale.ENGLISH).format(dow)); // Saturday
        out.println(DateTimeFormatter.ofPattern("EEE", Locale.ENGLISH).format(dow)); // Sat
        out.println(DateTimeFormatter.ofPattern("EEEEE", Locale.ENGLISH).format(dow)); // S
    }

    private static DayOfWeek getFirstDayOfMonth(int year, int month) {
        return YearMonth.of(year, month)
                        .atDay(1)
                        .getDayOfWeek();
    }
}

ONLINE DEMO

Note: If your requirement is to find the same for a specific timezone, use LocalDate#atStartOfDay(ZoneId) as shown below:

import static java.lang.System.out;

import java.time.DayOfWeek;
import java.time.YearMonth;
import java.time.ZoneId;

public class Main {
    public static void main(String[] args) {
        DayOfWeek dow = getFirstDayOfMonth(2021, 5, ZoneId.of("America/Los_Angeles"));
        out.println(dow); // SATURDAY
    }

    private static DayOfWeek getFirstDayOfMonth(int year, int month, ZoneId zoneId) {
        return YearMonth.of(year, month)
                        .atDay(1)
                        .atStartOfDay(zoneId)
                        .getDayOfWeek();
    }
}

ONLINE DEMO

Learn more about java.time, the modern Date-Time API^* from Trail: Date Time.

---

^{* For any reason, if you have to stick to Java 6 or Java 7, you can use ThreeTen-Backport which backports most of the java.time functionality to Java 6 & 7. If you are working for an Android project and your Android API level is still not compliant with Java-8, check Java 8+ APIs available through desugaring and How to use ThreeTenABP in Android Project.}

Answer 4

construct a calendar (cal) with the given month and year, and day 1, and cal.get(Calendar.DAY_OF_WEEK)... sorry, don't think there are any builtins just for that task