Abstraction in Java is a technique for hiding complex implementation details and presenting a simplified interface to the user. It focuses on showcasing essential functionality while concealing internal workings. For example, an ATM machine only exposes basic operations like withdrawal, balance check, and cancellation, hiding the intricate internal mechanisms. This simplifies user interaction. Abstraction can be achieved in Java using abstract classes and interfaces. Abstract classes provide a blueprint for subclasses, defining methods that need to be implemented by the subclasses. Interfaces define methods that any class implementing the interface must provide, promoting a common set of behaviors. By leveraging abstraction, developers can create modular, reusable code, making it easier to maintain and understand complex systems.