Why does the Calendar class not have a public constructor?

Question

Will new Calendar() have any difference from Calendar.getInstance()?

The question is just as simple as it is........... Since the system refuse to post it, I just copy some nonsense here.

private static Calendar calendar = Calendar.getInstance();
public static int getCalendar(long time, int calendarConst) {
    calendar.setTimeInMillis(time);
    return calendar.get(calendarConst);
}

Answer 1

I think the answer is simple Calendar is an Abstract class, so we cant create an instance of it. Now when you call Calendar.getInstance then GregorianCalendar instance is created.

Answer 2

There is a detailed account of the difference between constructors and static factory methods in Effective Java second edition, which is well worth a read.

The main difference here is in what the two return:

• The constructor always returns a non-null instance of the specific class on which the constructor is invoked, or it throws an exception and you don't get a reference to the part-constructed instance;
• A static factory method can return a subclass or null (or it throws an exception).

The latter case is exactly what Calendar.getInstance() does: you clearly don't get back an instance of Calendar itself, since it is abstract, but instead you might get a GregorianCalendar, JapaneseImperialCalendar etc.

This decouples Calendar's implementation from the client code: you can make changes to Calendar without the client needing to make changes.