Takes an input year and returns whether the year is a leap year.

IsLeapYear.java

public class NumberOfDaysInMonth {

    public static boolean isLeapYear(int year){
        boolean isLeapYear = false;

        if (year < 1 || year > 9999){
            return false;
        }
        
        if (year % 4 == 0){
            if (year % 100 == 0){
                isLeapYear = false;
            } else {
                isLeapYear = true;
            }
        }
        if (year % 400 == 0) {
            isLeapYear = true;
        }

        return isLeapYear;
    }

    public static int getDaysInMonth(int month, int year){
        int days = 0;

        if (month < 1 || month >12){
            return -1;
        }
        if (year < 1 || year > 9999){
            return -1;
        }
        if (isLeapYear(year)){
            switch (month){
                case 1: case 3: case 5: case 7: case 8: case 10: case 12:
                    days = 31;
                    break;
                case 2:
                    days = 29;
                    break;
                case 4: case 6: case 9: case 11:
                    days = 30;
                    break;
            }
        } else {
            switch (month){
                case 1: case 3: case 5: case 7: case 8: case 10: case 12:
                    days = 31;
                    break;
                case 2:
                    days = 28;
                    break;
                case 4: case 6: case 9: case 11:
                    days = 30;
                    break;
            }
        }

        return days;
    }
}