What is the Difference Between Interfaces and Abstract Classes?

1. Purpose Differences:

   • Interface: Define a contract that implementing classes must adhere to, primarily used to establish a contract between objects.
   • Abstract Class: Primarily used to provide common, predefined states (variables) or behaviors (methods) for a group of classes, with some behaviors implemented through abstract methods to achieve polymorphism.

2. Implementation Inheritance vs. Interface Inheritance:

   • Interface: Can only declare methods and constants; they cannot implement methods (though since Java 8, interfaces can include default and static methods).
   • Abstract Class: Can declare and implement methods, including concrete methods (i.e., methods that are fully implemented).

3. Constructors:

   • Interface: Cannot contain constructors.
   • Abstract Class: Can contain constructors to initialize basic states of the class.

4. Multiple Inheritance:

   • Interface: A class can implement multiple interfaces, supporting multiple inheritance.
   • Abstract Class: A class can inherit only one abstract class, not supporting multiple inheritance.

5. Access Modifiers:

   • Interface: Default methods and variables are public.
   • Abstract Class: Can have public, protected, and private methods and variables.

In summary, both interfaces and abstract classes have specific use cases; the choice depends on specific requirements. Interfaces are better suited for defining contracts between different classes, while abstract classes are better for providing common code and abstract methods for a group of related classes.