Question

I’m making a small project for my university which is supposed to enroll students into different courses. I do have one issue though, each course has start/end enrollment day. How can I now use Date/Calendar to avoid having to make my own method. I will need two though, one is setDates(), used to set start and end dates for enrollment, and second is isOpen(), which will return error if a student tries to enroll too early or too late. (Assuming the moment of applying is the moment the program is run, so basically “now”)

Answer 1

isOpen() can be this simple:

public boolean isOpen() {
    Date now = new Date();
    return !now.before(startDate) && !now.after(endDate);
}

setDates() can just be a simple setter (though you should protect the invariant that end date cannot be before start date)

private Date startDate, endDate;

public void setDates(Date startDate, Date endDate) {
    Date startCopy = new Date(startDate);
    Date endCopy = new Date(endDate);
    if ( startCopy.after(endCopy) ) {
        throw new IllegalArgumentException("start must not be after end");
    }

    this.startDate = startCopy;
    this.endDate = endCopy;
 }

This type of logic is very common though and the third-party Joda-Time library does a very good job of encapsulating it for you (e.g. through the Interval class and its containsNow() method).