What is Abstract class and interface?

Abstract Class: An abstract class is a class that is declared abstract, it may or may not include abstract methods. When an abstract class is subclassed, the subclass usually provides implementations for all of the abstract methods in its parent class. However, if it does not, then the subclass must also be declared abstract.

Interface: An interface is a reference type in Java. It is similar to a class. It is a collection of abstract methods. A class implements an interface, thereby inheriting the abstract methods of the interface. Along with abstract methods, an interface may also contain constants, default methods, static methods, and nested types.