Understanding Abstract Classes and Interfaces in Java

Abstract classes in Java can't be instantiated. They are templates for child classes, enforcing a contract. Unlike regular classes, they can have abstract methods that must be implemented by subclasses.

Interfaces in Java define methods without bodies, acting purely as a blueprint for classes. Classes can implement multiple interfaces, promoting flexibility and separation of concerns unlike single inheritance from abstract classes.

Abstract classes can have member variables and concrete methods. Interfaces initially couldn't, but since Java 8, they can have default and static methods, blurring the line between them and abstract classes.

Use abstract classes when subclasses share a common structure or behavior. Use interfaces for different classes that don't share ancestry, to guarantee they implement certain methods, fostering polymorphism.

Java 8 introduced default methods in interfaces, allowing implementation of methods without breaking existing classes implementing the interface. They provide flexibility to evolve interfaces while maintaining backward compatibility.

Static methods in interfaces, added in Java 8, are not inheritable like static methods in classes. They help in providing utility methods related to an interface, without forcing implementation in the class.

Marker interfaces have no methods and serve to indicate metadata about a class. For example, Serializable interface in Java signals to the JVM that objects of a class can be serialized.