Abstraction is a fundamental concept in object-oriented programming that allows us to hide complex implementation details and only expose relevant information to the user. Abstraction is achieved in Java through the use of abstract classes and interfaces.

An abstract class is a class that cannot be instantiated, i.e., we cannot create objects of that class. It can contain both abstract and non-abstract methods. An abstract method is a method without any implementation, i.e., it has no body. Any class that extends an abstract class must define (implement) all the abstract methods of that class.

Interfaces are similar to abstract classes in that they cannot be instantiated. An interface only contains method signatures (without any implementation). Classes can implement multiple interfaces, and any class implementing an interface should provide implementation to all the methods of that interface.

Abstraction provides us the flexibility to change the implementation of our code without affecting the user. We can modify the underlying implementation of a class without changing its external interface (i.e., the public methods visible to the user). Abstraction also makes our code more modular and easier to maintain.

In summary, abstraction is a powerful technique in Java programming that allows us to create more flexible and modular code by hiding implementation details from the user.