Instance of an abstract class

[MrKamal Joshi]

How the instance of an abstract class is created?As there exist a getInstance() method for an abstract class java.util.Calender.

[Vijay Tidake]

Hi,

Although Calender class has a abstract method getInstance(), but when you say Calendar calc=Calendar.getInstance();

calc is referring to the class instance of class GregorianCalendar as "GregorianCalendar extends Calendar "

Thanks

[Matthew Brown]

If you ever want to know the exact class that is returned (and usually you don't need to know - that's the point of polymorphism - but it might be interesting) then just print out the value of theObj.getClass().getName().

[Aniela Kącka]

Hi...

I'm not sure, if really calling the getInstance() static method of the Calendar class, you really make an instance of GregorianCalendar. However abstract class can never be directly instantiated, there is something called "annonymous inner type", which allows you to create a no-name subclass of the abstract class and an instance of this:

Calendar cal = new Calendar() {
				
				@Override
				public void roll(int field, boolean up) {
					// TODO Auto-generated method stub
					
				}
				
				@Override
				public int getMinimum(int field) {
					// TODO Auto-generated method stub
					return 0;
				}
				
				@Override
				public int getMaximum(int field) {
					// TODO Auto-generated method stub
					return 0;
				}
				
				@Override
				public int getLeastMaximum(int field) {
					// TODO Auto-generated method stub
					return 0;
				}
				
				@Override
				public int getGreatestMinimum(int field) {
					// TODO Auto-generated method stub
					return 0;
				}
				
				@Override
				protected void computeTime() {
					// TODO Auto-generated method stub
					
				}
				
				@Override
				protected void computeFields() {
					// TODO Auto-generated method stub
					
				}
				
				@Override
				public void add(int field, int amount) {
					// TODO Auto-generated method stub
					
				}
			};

As you can see, you must then implement all the abstract methods from the abstract superclass (Calendar in this case).

I think the getInstance() method of Calendar class might work like that.

[Vijay Tidake]

Hi,

If you look at the source of getInstance() will you see

 public static Calendar getInstance()
    {
        Calendar cal = createCalendar(TimeZone.getDefaultRef(), Locale.getDefault());
	cal.sharedZone = true;
	return cal;
    }

 private static Calendar createCalendar(TimeZone zone,
					   Locale aLocale)
    {
	// If the specified locale is a Thai locale, returns a BuddhistCalendar
	// instance.
	if ("th".equals(aLocale.getLanguage())
	    && ("TH".equals(aLocale.getCountry()))) {
	    return new sun.util.BuddhistCalendar(zone, aLocale);
	} else if ("JP".equals(aLocale.getVariant())
		   && "JP".equals(aLocale.getCountry())
		   && "ja".equals(aLocale.getLanguage())) {
	    return new JapaneseImperialCalendar(zone, aLocale);
	}	    

	// else create the default calendar
        return new GregorianCalendar(zone, aLocale);	
    }

Hope this will clear your confusion

Thanks

[Joanne Neal]

Vijay Tidake wrote:If you look at the source of getInstance() will you see

And this is a perfect example of polymorphism in action. I don't know which version of the source code Vijay looked at but I looked at the 1.6_20 source code and it's createcalendar method looked like this

    private static Calendar createCalendar(TimeZone zone,
					   Locale aLocale)
    {
	// If the specified locale is a Thai locale, returns a BuddhistCalendar
	// instance.
	if ("th".equals(aLocale.getLanguage())
	    && ("TH".equals(aLocale.getCountry()))) {
	    return new sun.util.BuddhistCalendar(zone, aLocale);
	}

	// else create the default calendar
        return new GregorianCalendar(zone, aLocale);	
    }

So, the code has been changed but anyone who uses the Calendar.getInstance method doesn't need to worry about it or make any changes to their source code to handle it.

[Aniela Kącka]

Yeap, it did, thanks

[Vijay Tidake]

Hi Joanne Neal,

I don't know which version of the source code Vijay looked

Im using jrockit-jdk1.6.0_20-R28.1.0-4.0.1

Thanks

[Ulf Dittmer]

The crucial point is that "Calendar.getInstance()" does NOT return an instance of the Calendar class - rather it returns whatever the getInstance mathod creates. That can't be an instance of Calendar, because that is abstract - it must be an instance of a subclass of Calendar that is not abstract. Mathew mentioned how you can determine what actual class that is.

[Maneesh Godbole]

Vijay Tidake wrote:
Although Calender class has a abstract method getInstance(), but when you say Calendar calc=Calendar.getInstance();
calc is referring to the class instance of class GregorianCalendar as "GregorianCalendar extends Calendar "

Calendar#getInstance() is not abstract, it is static. Two different things. The Calendar class is abstract.

Vijay Tidake wrote:
calc is referring to the class instance of class GregorianCalendar as "GregorianCalendar extends Calendar "

Not always. As quoted from the API Doc

Gets a calendar using the default time zone and locale.

If you actually look at the comment in the code snippet you posted, it can also return the Buddhist and JapaneseImperialCalendar which are not Gregorian

[Vijay Tidake]

Hi Maneesh,

Absolutely right.

Thanks