abstract methods are not implemented
(not even a default one).
This is better than putting in a dummy
procedure as a placeholder.
Derived
classes must eventually implement them;
if they
dont then they must be abstract classes themselves.
Overriding is resolved at runtime.
Abstract class is one that contains an abstract method;
need to be explicitly declared as
such.
Abstract classes may have non-abstract methods & static
fields.
Abstract classes cannot be created (no constructor),
except using super()