You can't instantiate an interface. Whatever you can do with an interface, you can do with an abstract class, too - such as using it as the base for an anonymous inner class. (Well, short of inheriting more than one of it, of course.)
The soul is dyed the color of its thoughts. Think only on those things that are in line with your principles and can bear the light of day. The content of your character is your choice. Day by day, what you do is who you become. Your integrity is your destiny - it is the light that guides your way. - Heraclitus
Just few words... An abtract class is a normal one, but additionnaly the class' designer declare that nobody can instanciante it. Abstract classes are used to describe basic concepts of an OO hierarchy. This is mainly a convenient way to declare general services a classes hierarchy provide.
So it doesn't make sense to instanciate an abtract class, nor to try to modify a constant(basically a variable you state to be unmodifiable)
Often an abstract class also has abstract methods (i.e. methods that are declared but have no body). It just wouldn't make sense to allow you to instantiate these since some of the functionality is missing.