Question

I’m trying to generate a random month and year with an upper bound of the current year and month and a lower bound that will be a static month and year.

I can generate the year with:

    int randomYear = (int) (Math.random() * ( currentYear - 2000 ))+2000;
    //returns a val 2000-2012

Where 2000 is my lower bound. So hypothetically that would generate a year between 2012-2000. The problem I run into is when I try to set a month I’m not sure how to handle if the random generator returns 2012 and there is only 2 months. Currently I have:

    int randomMonth = (int) Math.random() * (12);

How do I handle the special cases such as the case with 2012 or if I set the lower bounds for a year 2000 and a month of 10?

Answer 1

This might seems a little bit long to you, but it’s much nicer to work with actual dates:

public static void main(String[] args) {
    Calendar now = Calendar.getInstance();
    Calendar min = Calendar.getInstance();
    Calendar randomDate = (Calendar) now.clone();
    int minYear = 2012;
    int minMonth = 2;
    int minDay = 18;
    min.set(minYear, minMonth-1, minDay);
    int numberOfDaysToAdd = (int) (Math.random() * (daysBetween(min, now) + 1));
    randomDate.add(Calendar.DAY_OF_YEAR, -numberOfDaysToAdd);

    DateFormat dateFormat = new SimpleDateFormat("yyyy/MM/dd");
    System.out.println(dateFormat.format(now.getTime()));
    System.out.println(dateFormat.format(min.getTime()));
    System.out.println(dateFormat.format(randomDate.getTime()));
}

public static int daysBetween(Calendar from, Calendar to) {
    Calendar date = (Calendar) from.clone();
    int daysBetween = 0;
    while (date.before(to)) {
        date.add(Calendar.DAY_OF_MONTH, 1);
        daysBetween++;
    }
    System.out.println(daysBetween);
    return daysBetween;
}

//edit: puh.. harder than I thought 😉 But that’s it.