Abstract Classes & Interfaces

Abstraction is the OOP principle of hiding the implementation details from the user and only showing the functionality. In Java, this is primarily achieved using abstract classes and interfaces. An `abstract class` is a class that is declared with the `abstract` keyword and cannot be instantiated. It serves as a blueprint for other classes. It can contain both abstract methods (methods without a body) and concrete methods (regular methods with a body). A class that extends an abstract class must provide an implementation for all of its abstract methods, or it must also be declared abstract. This is useful when you want to provide some common, already implemented functionality in a base class but force subclasses to provide their own implementation for certain other methods. An `interface`, on the other hand, is a completely abstract type. It can only contain method signatures (implicitly `public` and `abstract`) and constant declarations (implicitly `public`, `static`, and `final`). A class can `implement` one or more interfaces, thereby agreeing to provide an implementation for all the methods defined in those interfaces. This is how Java achieves a form of multiple inheritance. The key difference is that an abstract class can have state (instance variables) and implemented methods, representing an 'is-a' relationship with some shared implementation, while an interface defines a 'can-do' relationship, a contract of capabilities, without any implementation details.