How is an interface different from an abstract class, or any class?

Before comparing the differences, it is essential to understand the definitions of an interface, abstract class, and a regular class. - Interface: An interface is a collection of abstract methods (methods without a body) that can be implemented by any class. It defines a contract specifying what methods a class must have, but not how those methods should be implemented. - Abstract class: An abstract class is a class that cannot be instantiated; it can only be inherited by other classes. It can contain both abstract methods (without an implementation) and regular methods (with implementations). - Regular class: A regular class is a blueprint for creating objects and can be instantiated. It defines the properties and methods that an object will have and can have implementations for all its methods.

- An interface only contains abstract methods, while an abstract class can have both abstract and regular methods. - A class can implement multiple interfaces, but it can inherit only from a single abstract class. - Interface methods must be public, whereas abstract class methods can have any access level (public, protected or private). - Interfaces don't have any instance variables (fields), while abstract classes may have instance variables. - Interfaces are more suitable for defining contracts, while abstract classes are better for sharing functionality among related classes.

- A regular class can be instantiated, while an abstract class and an interface cannot be instantiated directly. An object of a class implementing an interface or inheriting an abstract class can be created. - A regular class provides full implementation for all its methods, whereas abstract classes and interfaces can have methods without implementation. - A regular class focuses on creating objects with specific properties and methods, while interfaces and abstract classes define templates for other classes to follow and use. In conclusion, interfaces are mainly used for defining contracts or behaviors that classes must implement, abstract classes are used for sharing common functionality among related classes, and regular classes are used for creating objects with specific properties and methods.