A variable defined as an interface or a class (whether abstract or not)
can reference any object instantiated from that class, or a descendant
thereof.

Hashtable = new Properties();//no problem.

Functions work exactly the same.  Consider a method in the Collections
class:

List synchronizedList(List list)

List is an interface.  Any object might be returned, with the sole
stipulation that it implements the LIst interface.

Walter Gildersleeve
Freiburg, Germany

______________________________________________________
http://linkfrog.net
URL Shortening
Free and easy, small and green.

They fall under the same rules as for other methods :

interface Base {
 public Map func();
}

interface Derived extends Base {
 public SortedMap func();
}

That is perfectly legal.  Your derived class's covariant method simply
can't break the contract of the overridden class/interface, ie, it can
only return something as restrictive or more restrictive as the base
class.

Walter Gildersleeve
Freiburg, Germany

______________________________________________________
http://linkfrog.net
URL Shortening
Free and easy, small and green.

On Tue, 29 Nov 2005 16:58:19 +0100, Joost Kraaijeveld
<J.Kraaijeveld@Askesis.nl> wrote, quoted or indirectly quoted someone
who said :

yes.  All you need is for the object you return to extend that
abstract class or implement that interface.  Sun does this sort of
thing all the time.

Have a look at URL.openConnection which nominally returns an
URLConnection but actually returns an HttpURLConnection for http:
urls.