Polymorphism is a concept in object-oriented programming that allows different objects to be represented as instances of a single class with a common interface. In Java, polymorphism is achieved through method overriding and method overloading.

Method overriding is when a subclass provides its own implementation of a method that is already defined in its super class. This allows the subclass to take on its own unique behavior while still inheriting functionality from its parent.

Method overloading is when a class has multiple methods with the same name but different parameters. This allows the class to handle different data types or variations of inputs in the same method.

Polymorphism is useful in situations where there are varying implementations of a particular behavior, but they share a common purpose or interface. By using a single class to represent all these implementations, code can be written that is more flexible and easier to maintain.