Class Fundamentals

Abstract

The first object-oriented programming languages, Simula67 (36] and Smalltalk [40], were based on the concepts of class and instance. The majority of the object-oriented languages now in use are based upon these two concepts. Indeed, we can correctly refer to these languages as promoting class-based programming, a style of program construction based upon the idea that the programmer first defines a collection of classes and instantiates those classes when required. Classes represent the primary concepts employed in the program and instances represent particular exemplars of those concepts. The concept of the class is very similar to that of the abstract data type, and the two are frequently identified; when constructing a class-based program, the programmer identifies complex (abstract) data types and uses them to structure the program. The identification of classes with types implies that class-based languages have extensible type systems or, when dynamically typed like Smalltalk or Lisp, possess extensible structuring methods.