Posted 16 years ago on 1/23/2008 and updated 3/8/2009
Take Away:
An abstract class member is a member that is specified in a class but not implemented. Classes that inherit from the class will have to implement the abstract member. Abstract members are a technique for ensuring a common interface with descendant classes. An abstract class is a class you cannot instantiate. A pure abstract class is a class with only abstract members.
An abstract class member is a member that is specified in a class but not implemented. Classes that inherit from the class will have to implement the abstract member. Abstract members are a technique for ensuring a common interface with descendant classes. An abstract class is a class you cannot instantiate. A pure abstract class is a class with only abstract members.
UML Definition
In UML terms, you would probably use different words oriented toward analysis. One might define an abstract class as a "Reusable OO design for a component. Specifies the interface of a class and the tree of subclasses that can be derived from it. Specify behavior (not implementation). A class intended to be used by sub-typing, for the behavior it describes rather than by having instances."
Abstract Classes and Interfaces are Similar Concepts
Interfaces and abstract classes are similar concepts. In a class tree, inheritance, including abstract classes and members, is used to design classes vertically. You add functionality from top to bottom adding methods and properties to descendant classes. Interfaces allow you to design horizontally across your class tree. Interfaces allow you to enforce a common interface horizontally and/or randomly in your class tree. Abstract classes allow you to enforce a common interface vertically from the abstract class down.
Plato's Universal
Sometimes programmers refer to an abstract class as a universal which dates back to about 400 BC to Plato and his theory of Forms. For example, a beautiful rose is a copy or imitation of a rose template (or class) and implemented the universal Forms flowerness and beauty. The theory of forms was his way of explaining how the same universal term can refer to so many particular things or events. The beauty form (either an abstract class or an interface in programming), for example, can be applied to hundreds of particular things because these things have something in common, their resemblance to, or participation in, the Form beauty. An object is beautiful to the extent that it participates in the Idea, or Form, of beauty.
Everything in the world of space and time is what it is by virtue of its resemblance to, or participation in, its universal Form. The ability to define the universal term is evidence that one has grasped the Form to which that universal refers. Plato conceived the Forms as arranged hierarchically; the supreme Form is the Form of the Good (his base-class). As a programmer, you can think of your problem domain, your application, as a set of objects. You can practice Plato's Forms using inheritance, abstract classes, and interfaces. Understanding Plato's theory of Forms can help you with the concept of abstraction in your own application.
Abstract Versus Concrete
You can think of a concrete method as the fullfillment of an abstract method and you can think of a concrete class as the fulfillment of an abstract class. However, a concrete member or class is really just an implemented member or class. The abstraction of a member or class is always there even when it's not there in code. In your code, abstraction is either implied or explicit. The abstraction is implicit (only in design) for regular and virtual methods and it's explicit when you code an abstract class or method or an interface -- an interface is a type of abstraction.
In general, a concrete class is a class where all the members are implemented and the class can be instantiated. Any class with one or more abstract members is not a concrete class.
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