What is the difference between an interface and an abstract class in Java?

In Java, both interfaces and abstract classes are used to implement abstraction, a key concept in Object-Oriented Programming (OOP). Abstraction is the process of hiding the implementation details and showing only the necessary information to the users.

Here are the main differences between an interface and an abstract class:

• An interface is a pure abstract class, meaning that it only defines abstract methods and constants, whereas an abstract class can have both abstract and non-abstract methods and other members.
• A class can implement multiple interfaces but can only inherit from a single abstract class.
• Interfaces cannot have constructors or instance variables, but abstract classes can.
• Interfaces are used to define a contract or a set of rules that a class must follow if it implements that interface, whereas abstract classes are used to provide a base implementation that concrete classes can inherit and modify.
• Finally, interfaces are used for full abstraction, whereas abstract classes are used for partial abstraction.

So, which one should you use? The answer depends on your specific use case. If you want to provide a base implementation that can be inherited and customized by other classes, use an abstract class. If you want to define a contract that classes must follow, use an interface.