typeparameter

A type parameter is a placeholder for a type used in generic definitions. It allows a single generic definition to operate on multiple concrete types, improving code reuse and type safety by deferring information about the specific type until instantiation.

In most languages, type parameters are declared as part of a generic type, function, or method. For example, a class Box<T> declares a type parameter T; within the class, members can use the type T. When the generic is used, a concrete type is supplied as the type argument, such as Box<String> or Box<Integer>.

Type parameters often have constraints to restrict allowable types. For instance, a type parameter T might be constrained to extend a base class or implement an interface. Some languages support multiple constraints or specialized capabilities, such as requiring T to be comparable. Variance modifiers in some languages designate how subtyping relates to generic types, influencing assignability between different instantiations.

Runtime behavior varies by language. In languages with type erasure, such as Java, type parameters are not retained at runtime and type arguments may not be available during execution. In languages with reified generics or with templates, the actual types are preserved or generated, affecting runtime type information and performance characteristics.

Type parameters are typically scoped to their declaration and do not exist outside the generic definition. They enable common patterns such as generic collections, type-parameterized algorithms, and interface or API definitions that work across multiple types.