How can I find Saturdays and Sundays in A given month?

Question

I want find all Saturdays and Sundays in A given month. How can I do so?

Answer 1

The java.util Date-Time API and their formatting API, SimpleDateFormat are outdated and error-prone.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 Date-Time API:

import java.time.DayOfWeek;
import java.time.LocalDate;
import java.time.YearMonth;
import java.time.temporal.TemporalAdjusters;
import java.util.List;
import java.util.stream.Collectors;

public class Main {
    public static void main(String[] args) {
        // Test
        System.out.println(getWeekends(2));// All weekends of Feb in the current year
        System.out.println(getWeekends(2020, 2));// All weekends of Feb 2020
    }

    /*
     * All weekends (Sat & Sun) of the given month in the current year
     */
    static List<LocalDate> getWeekends(int month) {
        LocalDate firstDateOfTheMonth = LocalDate.now().withMonth(month).with(TemporalAdjusters.firstDayOfMonth());
        
        return firstDateOfTheMonth
                .datesUntil(firstDateOfTheMonth.plusMonths(1))
                .filter(date -> date.getDayOfWeek() == DayOfWeek.SATURDAY || date.getDayOfWeek() == DayOfWeek.SUNDAY)
                .collect(Collectors.toList());
    }

    /*
     * All weekends (Sat & Sun) of the given year and the month
     */
    static List<LocalDate> getWeekends(int year, int month) {
        LocalDate firstDateOfTheMonth = YearMonth.of(year, month).atDay(1);
        
        return firstDateOfTheMonth
                .datesUntil(firstDateOfTheMonth.plusMonths(1))
                .filter(date -> date.getDayOfWeek() == DayOfWeek.SATURDAY || date.getDayOfWeek() == DayOfWeek.SUNDAY)
                .collect(Collectors.toList());
    }
}

Output:

[2021-02-06, 2021-02-07, 2021-02-13, 2021-02-14, 2021-02-20, 2021-02-21, 2021-02-27, 2021-02-28]
[2020-02-01, 2020-02-02, 2020-02-08, 2020-02-09, 2020-02-15, 2020-02-16, 2020-02-22, 2020-02-23, 2020-02-29]

ONLINE DEMO

Non-Stream solution:

import java.time.DayOfWeek;
import java.time.LocalDate;
import java.time.YearMonth;
import java.time.temporal.TemporalAdjusters;
import java.util.ArrayList;
import java.util.List;

public class Main {
    public static void main(String[] args) {
        // Test
        System.out.println(getWeekends(2));// All weekends of Feb in the current year
        System.out.println(getWeekends(2020, 2));// All weekends of Feb 2020
    }

    /*
     * All weekends (Sat & Sun) of the given month in the current year
     */
    static List<LocalDate> getWeekends(int month) {
        LocalDate firstDateOfTheMonth = LocalDate.now().withMonth(month).with(TemporalAdjusters.firstDayOfMonth());
        List<LocalDate> list = new ArrayList<>();

        for (LocalDate date = firstDateOfTheMonth; !date
                .isAfter(firstDateOfTheMonth.with(TemporalAdjusters.lastDayOfMonth())); date = date.plusDays(1))
            if (date.getDayOfWeek() == DayOfWeek.SATURDAY || date.getDayOfWeek() == DayOfWeek.SUNDAY)
                list.add(date);

        return list;
    }

    /*
     * All weekends (Sat & Sun) of the given year and the month
     */
    static List<LocalDate> getWeekends(int year, int month) {
        LocalDate firstDateOfTheMonth = YearMonth.of(year, month).atDay(1);
        List<LocalDate> list = new ArrayList<>();

        for (LocalDate date = firstDateOfTheMonth; !date
                .isAfter(firstDateOfTheMonth.with(TemporalAdjusters.lastDayOfMonth())); date = date.plusDays(1))
            if (date.getDayOfWeek() == DayOfWeek.SATURDAY || date.getDayOfWeek() == DayOfWeek.SUNDAY)
                list.add(date);

        return list;
    }
}

ONLINE DEMO

Learn more about the modern Date-Time API from Trail: Date Time.

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^{* 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.}