An abstract class cannot have any objects or instances, but it
can have subclasses which may be abstract or concrete. UML uses
tag {abstract} below the name in the name compartment of
the class. Also the name is given in italic. An abstract class
has normally abstract operations. An abstract operation has only
signature but no implementing method in the class where the operation
is specified. In UML an abstract operations name is given in italic
and its signature is followed by the tag {abstract}. A class
with at least one abstract operation must be an abstract class. The
abstract operations are implemented in subclasses. Thus
drive() abstract operation of vehicle is implemented in its
subclasses car, boat, truck. Concrete classes can be instantiated into
objects. All their operations have implementations in the
class. However these operations may be redefined in its subclasses,
though their signatures must remain the same. All instances of a
subclass will use the operations (with their new definitions if
changed). Subclasses allow new attributes, operations and associations
to be added to those inherited from the superclass. This possibility
of using newly defined methods in subclasses for the operation
inherited from the superclass is called polymorphism.
Discriminators specify the basis for inheritance, e.g., the
characteristics inherited. For example vehicles may be subclassed on
the basis of methods used for propulsion or on the basis of number of
wheels. Java uses extend to create subclass.