What is the role of interfaces in OOP, and how do they support polymorphism?

Interfaces in OOP define a contract for classes to follow, supporting polymorphism by allowing objects to take many forms.

In Object-Oriented Programming (OOP), an interface is a reference type, similar to a class, that can contain only constants, method signatures, default methods, static methods, and nested types. It provides a way to ensure that a class adheres to a certain contract or behaves in a certain way. An interface is not a class, but rather a set of rules that classes can choose to follow by 'implementing' the interface.

The role of interfaces is crucial in supporting polymorphism, a core concept in OOP. Polymorphism, derived from Greek words meaning 'many shapes', is the ability of an object to take on many forms. The most common use of polymorphism in OOP occurs when a parent class reference is used to refer to a child class object. This is where interfaces come into play. When a class implements an interface, it inherits the abstract methods of the interface. This allows objects of the implementing class to be treated as objects of the interface.

For example, consider an interface 'Animal' with a method 'makeSound()'. Different classes like 'Dog', 'Cat', and 'Bird' might implement this interface and provide their own implementation of the 'makeSound()' method. Here, an object of 'Dog' can be treated as an 'Animal' and the 'makeSound()' method can be called, producing a different result depending on whether the 'Animal' is a 'Dog', 'Cat', or 'Bird'. This is polymorphism in action, facilitated by the use of interfaces.

In addition, interfaces also support the concept of multiple inheritance in languages that do not directly support this feature, such as Java. A class can implement multiple interfaces, thereby 'inheriting' the methods of all these interfaces. This allows for a form of multiple inheritance, where a class can have behaviour from multiple sources.

In conclusion, interfaces play a vital role in OOP by defining a contract for classes to follow and supporting polymorphism, allowing objects to take on many forms.