upcasts

An upcast is a cast or conversion of a value to a supertype in a type hierarchy, commonly described as a widening conversion. In object-oriented languages that support subtyping, upcasting often involves treating an instance of a subclass as an instance of its superclass or an implemented interface. The operation is usually implicit and safe, because the runtime type remains the same and all behavior defined by the supertype is preserved.

In practice, upcasting is used to enable polymorphism. For example in Java: Dog d = new Dog(); Animal a = d; The variable a uses the Animal type, so only methods declared in Animal are accessible, though the object is still a Dog and may override Animal methods.

Downcasting is the reverse and may require explicit casts; it can fail at runtime if the underlying object is not actually of the target subtype. Upcasting does not typically incur runtime checks, although some languages may insert checks for certain interfaces or generic types.

In type systems, upcasting corresponds to subtyping; it is fundamental to polymorphism, interfaces, and virtual method dispatch. When using generics or type erasure, upcasting behavior can be constrained by the language's rules. Some languages also permit upcasting through interface references, or to a base class such as Object in Java.