supertype

A supertype is a term used in type theory, object-oriented programming, and data modeling to describe a type that serves as a common ancestor for more specific types, called subtypes. A supertype defines the attributes and operations shared by its subtypes. Any value that belongs to a subtype is also a valid value of the supertype, a property known as subtyping or upcasting. The reverse operation, downcasting, may require runtime checks and can fail if the value does not actually conform to the subtype.

In programming languages, supertypes include classes and interfaces. Examples: Object is the universal supertype of all classes in Java; Number is a supertype of Integer and Double; an Animal supertype might organize subtypes such as Cat and Dog. Generics and polymorphism rely on supertype relationships to allow generic code to operate on any subtype.

In data modeling and conceptual design, supertypes model generalization: a supertype holds common attributes such as identifiers and shared relationships, while subtypes add specialized attributes. Subtypes can be disjoint or overlapping: disjoint means an instance belongs to at most one subtype, while overlapping allows multiple subtypes to apply to the same instance.

Implementation and design considerations include choosing when to use a supertype, ensuring the common interface is meaningful, and balancing reuse with maintainability. The approach supports polymorphism and code reuse but can introduce complexity if subtypes diverge or conflict in behavior.