Abstract Classes and Interfaces in OOP

Abstract Classes

• Abstract classes are blueprints for creating objects. They can contain both abstract methods (methods without implementations) and concrete methods (methods with implementations).
• Abstract classes cannot be instantiated directly. You must create a subclass that provides implementations for the abstract methods.
• Inheritance is used to create subclasses that inherit behavior from the abstract class.
• They support multiple functionalities compared to interfaces, as they can contain data members (variables) along with abstract and concrete methods.
• Abstract classes allow for a shared implementation among subclasses. This enhances code reusability and reduces redundancy.
• They are useful when you need a common base class with some pre-defined behavior and some behavior still requiring specification by subclasses.

Interfaces

• Interfaces define a set of methods that classes must implement.
• Interfaces contain only abstract methods (no concrete methods) and constants.
• Interfaces can have multiple implementations (more like contracts or blueprints) by different classes.
• Interfaces do not support inheritance of variables; only methods are involved.
• They specify a contract that different classes must fulfill, ensuring a common set of behavior between them.
• They promote loose coupling and flexibility, as classes can implement multiple interfaces.

Differences Between Abstract Classes and Interfaces

• Methods: Abstract classes can have both abstract and concrete methods, while interfaces can only have abstract methods.
• Variables: Abstract classes can have instance variables, while interfaces cannot.
• Inheritance: A class can extend only one abstract class but can implement multiple interfaces.
• Implementation: Abstract classes provide partial implementation, while interfaces define nothing but signatures.
• Focus: Abstract classes are focused on shared implementation and behavior, while interfaces are focused on defining a contract or set of methods.
• Multiple Inheritance: Abstract classes do not support multiple inheritance, while interfaces do.
• Default Methods: Interfaces can have default implementations for methods using default methods since Java 8, allowing for extending interface functionality without breaking existing code using the interface (i.e. interfaces can be updated and subclasses do not necessarily need updating immediately when adding a method). Abstract classes do not have the same default method support; this is handled through the extending class.
• Use Cases: A class that shares a lot of common actions is better defined as an abstract class whereas an interface is useful when you want to specify common actions that can be implemented in classes whose primary functionality are significantly different; this encourages loose coupling.
• Relationship: The relationship between a class and an abstract class can be considered "is-a" while a relationship between a class and an interface is a "can-do" or "has-a" relationship.