difference between abstract class and interface

In object-oriented programming, both abstract classes and interfaces provide a way to define a set of methods that must be implemented by a subclass or a implementing class. Here are the key differences between abstract classes and interfaces:

Definition: An abstract class is a class that cannot be instantiated and contains one or more abstract methods, which are methods that have no implementation. An interface, on the other hand, is a collection of abstract methods and constants that can be implemented by any class.

Inheritance: An abstract class can be extended by a subclass, which must implement all the abstract methods defined in the abstract class. A class can implement multiple interfaces, but it cannot extend more than one abstract class.

Access modifiers: An abstract class can have public, private, protected or package-level access modifiers on its methods and fields. In contrast, all the methods defined in an interface are public by default.

Implementation: An abstract class can have both abstract and non-abstract methods, and it can also have instance variables. In contrast, all the methods defined in an interface are abstract, and it cannot have instance variables.

Extensibility: An abstract class can be extended to add new functionality, but this can break the existing code if the subclasses depend on the original functionality. Interfaces, on the other hand, are more extensible because new methods can be added to an interface without affecting the existing code.

Implementation details: An abstract class can have implementation details for some of its methods, while an interface only declares the method signatures without providing any implementation details. This means that an abstract class can provide default implementations for some of its methods, which the subclass can either use or override. An interface, on the other hand, cannot provide any implementation details.

Purpose: Abstract classes are used when a base class needs to provide some implementation details, but the derived classes need to provide their own implementation details for some methods. Interfaces, on the other hand, are used to define a contract that must be followed by all implementing classes, regardless of their implementation details.

Multiple inheritance: Some programming languages allow multiple inheritance of interfaces, which means that a class can implement multiple interfaces. However, multiple inheritance of classes can lead to the diamond problem, where the inheritance hierarchy becomes ambiguous. In contrast, abstract classes can provide a way to simulate multiple inheritance by allowing a subclass to extend multiple abstract classes.

Constructor: An abstract class can have a constructor, which is called when an instance of the subclass is created. In contrast, an interface cannot have a constructor, because it cannot be instantiated.

In summary, both abstract classes and interfaces are used to define a set of methods that must be implemented by a subclass or a implementing class, but they have some key differences in terms of inheritance, access modifiers, implementation details, purpose, multiple inheritance, and constructors. The choice between an abstract class and an interface depends on the specific requirements of the application and the design of the class hierarchy.