Abstract Class   An abstract class is a class that cannot be directly instantiated. . It cannot have attributes or methods and but can contain abstract operations. It is semantically equivalent to an interface, and can participate in relationships such as generalization dependencies, and association with some restrictions.

Explanation

An abstract class is a type of class that can be identified by its italicized name and operations. It can be used to give a formal specification of the services of the class without specifying the way the services are provided. An abstract class cannot have attributes and is said to be abstract because it cannot have direct instances. This means that a class instance (object) can never be a direct instance of an abstract class. A class instance (object) may be an indirect instance in that it may be an instance of a sub-class (child) of the abstract class. In this circumstance the sub-class will have "concrete" operations. It is equivalent to an interface and has the same relationships and restrictions of an interface.

Important Points

		All operations must be abstract		The operation compartment contains an ordered list of operations usually one to a line all of which must be abstract.
		An abstract class cannot have attributes		An abstract class cannot contain attributes, nor can it have direct instances. They can participate in relationships like any class, however there are some restrictions in the way these relationships are applied.

Related Entries

Class  Graphically Similar,  Used Together A class is related to an abstract class in that they have similar semantics. The abstract class is more restrictive than a class and does not allow attributes The operations are different to those of a class, in the sense that they are abstract and cannot have methods.