Is polymorphism also present in method overloading?

Question

For a long time I thought polymorphism was defined as one class inheriting from another, such that the inheriting class is of a more narrow type than the super class. Recently however, I've been told that method overloading is also called polymorphism. I am currently under the impression that method overriding and method overloading are respectively called runtime and compile time polymorphism.

My questions are as follows:

1. Am I right in thinking of method overriding and method overloading as runtime and compile time polymorphism?
2. Do you need to override anything in order to overload a method?

Answer 1

The first thing, overriding and overloading, both are different. You are correct, overloading is termed as static or compile time polymorphism. At the compile time itself the Java compiler binds the method calls to respective callers and hence the term. Where as in dynamic or runtime polymorphism, binding is deferred till the execution. We can execute runtime polymorphism using inheritance.

Answer 2

Overloading is indeed a form of polymorphism! Albeit one that is not related to Subtyping and one that is not often discussed, or even acknowledged, in Java OOP circles¹ - but we can change that!

..polymorphism is the provision of a single interface to entities of different types.

That is, overloading is compile-time Ad hoc (Overloading) Polymorphism at work.

In programming languages, ad hoc polymorphism is a kind of polymorphism in which polymorphic functions can be applied to arguments of different types, because a polymorphic function can denote a number of distinct and potentially heterogeneous implementations depending on the type of argument(s) to which it is applied. [ad hoc polymorphism] is also known as function overloading or operator overloading.

An overloaded method might also participate in run-time Subtype (Inclusion) Polymorphism - realized in Java through method overriding, interfaces, and virtual dispatch.

---

See these additional questions; the selection is based upon questions with [detailed] answers that [mostly] support the case argued for above..

• Is polymorphism possible without inheritance? (I'm voting to close as a duplicate of this question.)
• Parametric polymorphism vs Ad-hoc polymorphism
• Is Method Overloading a Type of Polymorphism?
• Overloading is compile-time polymorphism. Really?
• Default arguments in function and polymorphism
• Is Method Overloading considered polymorphism?
• Confusion with polymorphism: Parametric, Inclusion, Coercion, and Overloading

Other resources; again selected to support the case above..

• On Understanding Types, Data Abstraction, and Polymorphism (Luca Cardelli, Peter Wegner 1985), a whitepaper from several years before Java, which builds on top of Stachey's paper.
• Fundamental Concepts in Programming Languages (Christoper Strachey 1967) which introduces such a polymorphism classification, but predates the use of "overloading".
• c2: Ad Hoc Polymorphism
• Lecture Notes on Polymorphism (CMU 15-411: Compiler Design, Frank Pfenning)

---

¹The Java Tutorial: Polymorphism makes no reference to overloading polymorphism; but neither does it mention that interfaces are sufficient for (or imply) subtype polymorphism in Java.