Strong typing of object-oriented languages revisited

This paper is concerned with the relation between <italic>subtyping</italic> and <italic>subclassing</italic> and their influence on programming language design. Traditionally subclassing as introduced by Simula has also been used for defining a hierarchical type system. The type system of a language can be characterized as <italic>strong</italic> or <italic>weak</italic> and the type checking mechanism as <italic>static</italic> or <italic>dynamic</italic>. Parameterized classes in combination with a hierarchical type-system is an example of a language construct that is known to create complicated type checking situations. In this paper these situations are analyzed and several different solutions are found. It is argued that an approach with a combination of static and dynamic type checking gives a reasonable balance also here. It is also concluded that this approach makes it possible to base the type system on the class/subclass mechanism.