Polymorphism

Polymorphism refers to the ability of executing different operations in response to the same message. Polymorphism can also be static or dynamic. In dynamic polymorphism the response to the message is decided on run-time while in static polymorphism it is decided out of run-time (i.e. on compile-time). The mechanism used to link the message with the method in dynamic polymorphism is known as late or dynamic binding while if the linking is done on compile time it is known as early or static binding.

Polymorphism is implemented using four different mechanisms:

• Overloading: an operation name refers to two different method implementations that differs either in its scope or signature. It uses early binding.
• Generics: also called templates, they define a way to declare generic classes parameterizing some of its types. It uses early binding.
• Overriding: a derived class re-defines a method inherited from the base class. It may use late binding.
• Polymorphic variable: also called assignment polymorphism, it refers to a variable that can have different types during execution. By performing an upcast we convert a variable referring to an object of a derived class to one referring to its base class. It is the most common situation for using a polymorphic variable as we can ensure that a derived object can always be treated as one of its base class. On the other hand if we convert an object of base class to one of its derived class we are performing a downcast. In this case we cannot ensure the correctness of the operation unless the variable had been previously upcasted. Assignment polymorphism is known as pure polymorphism if the variable is used as a parameter of an operation. It uses late binding.