Types and Subtypes, Classes and Subclasses

  The notions of type and class are often confounded in object-oriented programming languages. In fact they play quite distinct roles, and can be usefully distinguished from each other. Types provide interface information that determines when certain operations are legal, while classes provide implementation information including initial values for instance variables and the bodies of methods. In the first subsection we discuss types and subtypes, while in the following subsection we discuss the notions of class and subclass.