Noninstantiable

Noninstantiable describes a type or entity that cannot be instantiated to produce objects. In software design, a noninstantiable type is typically used as a namespace for related functions and constants, rather than as a blueprint for objects. The intent is to prevent object creation while still providing a cohesive grouping of behavior.

Implementation varies by language. In Java, a noninstantiable utility class is usually declared final and provides a private constructor, ensuring that no code can call new on it. All members are static. A well-known example is the Math class, which exposes static mathematical functions but cannot be instantiated. In C#, a noninstantiable construct is often a static class, which enforces that the class cannot be instantiated and can only contain static members. In other languages, a private constructor or other access control can serve the same purpose. Some languages provide dedicated language features for noninstantiable types.

Design considerations and alternatives: Use when grouping stateless helpers; avoid when you need polymorphism or stateful behavior; be aware that reflection can bypass access restrictions in some environments; consider modules or namespaces instead to avoid artificial types. In addition, some languages distinguish between noninstantiable and abstract types; abstract classes may still be instantiable via subclassing, while noninstantiable patterns prevent direct instantiation.

Noninstantiable types also appear in type theory to denote types with no inhabitants, such as the empty type, illustrating that some concepts resemble noninstantiability beyond code.