Polymorphism – AP Computer Science A College Board Revision

Polymorphism is a fundamental concept in object-oriented programming. It allows objects of different types to be treated as objects of a common superclass.
Polymorphism enables an object to decide which form of a method to implement at runtime based on its type.
The name ‘polymorphism’ comes from the Greek words ‘poly’ meaning ‘many’, and ‘morph’ meaning ‘form’. Thus, polymorphism allows for many forms.
Polymorphism is the ability of a single interface to represent a variety of different classes and objects. This is a fundamental aspect of inheritance.

With polymorphism, you can write code that does not need to know the exact type of the class or the objects that it deals with, only that they are subclasses of a certain superclass. This is known as loose coupling.
Polymorphism promotes flexibility and extensibility in code as objects of different classes can be treated uniformly.
It simplifies code and makes it more understandable and maintainable, since it promotes the use of a single, common interface.

Polymorphism in Java comes in two forms: compile-time polymorphism (also known as static binding or method overloading) and runtime polymorphism (also known as dynamic binding or method overriding).
Compile-time polymorphism is achieved by method overloading. In this case, the binding between the method call and the method to be executed is decided at compile time.
Runtime polymorphism is achieved by method overriding (redefining methods in derived classes that have already been defined in the base class). In this case, the binding between the method call and the method to be executed is decided at runtime.

In Java, you can demonstrate polymorphism by defining a method in a superclass and overriding it in a subclass. Then, you can use a reference variable of the superclass to refer to an object of the subclass and call the method.
If the method is overridden in the subclass, the subclass version will be executed. If it’s not overridden, the superclass version is executed. This decision is made at runtime, demonstrating runtime polymorphism.

The instanceof operator allows you to check whether an object is an instance of a certain class or implements a particular interface.
It can be used to ensure type compatibility before performing certain operations, providing a measure of safety when using polymorphism.

Sometimes, you may need to cast an object from a superclass type to a subclass type. This is known as downcasting.
Downcasting lets you access subclass-specific fields and methods that aren’t accessible from the superclass type.
However, downcasting can be risky. If the object being cast is not actually an instance of the subclass, a ClassCastException will be thrown. The instanceof operator can be used before casting to ensure type compatibility.