Project Euler #19 in Java

Question

You are given the following information, but you may prefer to do some research for yourself.

1 Jan 1900 was a Monday.
Thirty days has September,
April, June and November.
All the rest have thirty-one,
Saving February alone,
Which has twenty-eight, rain or shine.
And on leap years, twenty-nine.
A leap year occurs on any year evenly divisible by 4, but not on a century unless it is divisible by 400.

How many Sundays fell on the first of the month during the twentieth century (1 Jan 1901 to 31 Dec 2000)?

Solution :

My following logic gives me 173 Sundays , whereas there are 171 Sundays !! Where are the extra 2 days came from ?

public static void main(String args[]) {

    Date startDate = new Date(1901, Calendar.JANUARY, 01);
    Date endDate = new Date(2000, Calendar.DECEMBER, 31);

    checkSundays(startDate, endDate);
}

private static void checkSundays(Date start, Date end) {
    int dayNum;

    Calendar startDate = Calendar.getInstance();
    startDate.setTime(start);
    System.out.println(startDate.getTime());

    Calendar endDate = Calendar.getInstance();
    endDate.setTime(end);
    System.out.println(endDate.getTime());
    int count = 0;

    while (startDate.before(endDate)) {
        for (int i = 1; i < 13; i++) {
            dayNum = startDate.get(Calendar.DAY_OF_WEEK);
            if (dayNum == 1) {
                count++;
            }

            System.out.println(startDate.getTime());
            startDate.add(Calendar.MONTH, 1);

        }
            System.out.println("Count " + count);

    }
}

Answer 1

You can use probability and do this with just a calculator.

There's 100 years in a century, and every year, there are 12 first days of the months. Divide that by 7 and you have your answer.

It's cheap, but it works.

Answer 2

Your following code uses deprecated constructor of Date

Date startDate = new Date(1901, Calendar.JANUARY, 01);
System.out.println(startDate);

which is not proper, it prints

Thu Jan 01 00:00:00 IST 3801

So use Calendar to construct Date,

    Calendar startDateCal = createDateInstance(0,1901,1)

    Calendar endDateCal = createDateInstance(11,2000,13)

and a Factory method

public static Date createDateInstance(int month, int year, int date){
  Calendar cal= Calendar.getInstance();
  cal.set(Calendar.YEAR, year);
  cal.set(Calendar.MONTH, month);
  cal.set(Calendar.DATE, date);

  return cal.getTime();

}

See your working code