Re: a question about factory method
On 12/18/2014 11:04 AM, John wrote:
Thank you all for replies.
Can you elaborate a little more about
Its getInstance() methods usually return a GregorianCalendar instance,
but could (if Locales and so on were set up) return an instance of
MayanCalendar or JulianCalendar or BabylonianCalendar or whatever:
Classes whose names, whose very existence, might not have been known
when the calling code was written.
My guess is something like below. Is that correct? Thank you very much.
public class Calendar {
/**
* Static factory method returns an object of this class or subclass. for choice: 1 for GregorianCalendar, 2 for MayanCalendar , 3 for JulianCalendar
*/
public static Calendar getInstance(Date aDate, int choice) {
if(choice==1){
... //return a GregorianCalendar object
}
else if(choice==2) {
... //return a MayanCalendar object
}
else if(choice==3) {
... //return a JulianCalendar object
}
else {
return null;
}
}
...
}
How can I make it return MayanCalendar or JulianCalendar?
First, you'd need to write MayanCalendar and JulianCalendar
classes (they're not part of Java SE; I just made them up as
examples that someone might add someday). These new classes would
be subclasses of Calendar:
public class MayanCalendar extends Calendar { ... }
public class JulianCalendar extends Calendar { ... }
.... so that Calendar.getInstance() -- which must return a Calendar --
could legitimately return them.
Next, you'd have to make the new classes known to Java. I haven't
actually tried to do this, but it can (probably) be done somehow: All
the variants of Calendar.getInstance() eventually wind up with a
TimeZone and a Locale, and from these they're supposed to produce a
suitable Calendar. Java doesn't specify exactly how suitable Calendars
are found, but if you look at the source code for Calendar.java you'll
find that (in Sun/Oracle's implementation) the heavy lifting happens in
CalendarProvider and LocaleProviderAdapter classes. (These are not part
of Java SE, but part of Sun/Oracle's implementation of Java SE.)
I can't explore much further, because the JDK doesn't provide source
for those internal classes; all I can see is the compiled bytecode. But
I imagine there's a configuration file somewhere that describes the
available Locales and things, and (presumably) there's a way to say
"For the following Locale, use *this* Calendar subclass." Whether this
actually happens exactly this way with Calendar isn't crucial to the
issue of using a factory method; the point is that a factory method
*could* do something like this, and make a run-time choice of which
precise class it returns. A constructor couldn't.
--
esosman@comcast-dot-net.invalid
"Don't be afraid of work. Make work afraid of you." -- TLM