Difference Between Abstract Class and Interface ( With Table)

As we all know, abstraction refers to the interior implementation of the feature and solely showing the practicality to the users. i.e., what it works (showing, However, the interface offers complete abstraction in Java, which abstract categories cannot do.

Abstract Class vs Interface

 The main difference between Abstract Class and Interface is that the associate interface will extend another Java interface solely; the associate abstract category will develop another Java category and implement multiple Java interfaces. Members of a Java interface differ public by default. A Java abstract category will have category members like personal, protected, etc.

An abstract class is a class that has the abstract keyword in its declaration. At least one abstract method, i.e., methods without a body, should be present in abstract classes. It may have a variety of concrete methods.

You can use abstract classes to generate blueprints for concrete classes. However, the abstract method must be implemented by the inheriting class.

The interface is a blueprint for a class that can be used to create it. The interface has no concrete methods (methods that have code). An interface’s methods are all abstract methods.

It is not possible to create an interface. Classes that implement interfaces, on the other hand, can be instantiated. Instance variables are never used in interfaces. However, public static final variables (also known as constant class variables) can be used.

What is Abstract Class?

An abstract class is a fundamental concept in object-oriented programming (OOP) that serves as a blueprint or template for other classes. It is a class that cannot be instantiated and is meant to be extended or subclassed by other classes. Abstract classes define common attributes and methods that multiple related classes will share within an inheritance hierarchy.

Key characteristics and features of abstract classes include:

1. Cannot be Instantiated: An abstract class cannot be used to create objects or instances directly. Instead, it provides a framework for subclasses to inherit and implement its properties and methods.
2. May Contain Abstract Methods: Abstract classes include one or more abstract methods. Method declarations without any implementation details. Subclasses must provide concrete implementations for these abstract methods.
3. May Include Concrete Methods: Abstract classes can also have concrete (fully implemented) methods that subclasses can inherit. These methods can provide common functionality shared by all subclasses.
4. Used for Code Reusability: Abstract classes promote code reusability and the DRY (Don’t Repeat Yourself) principle by allowing common code to be defined in a single abstract class and shared among related subclasses.
5. Inheritance: Subclasses inherit the attributes and methods defined in an abstract class. This allows for specialization and customization of behavior while maintaining a common structure.
6. Abstract vs. Concrete Classes: Abstract classes are distinguished from concrete classes, which can be instantiated directly. Concrete classes provide full implementations for all their methods.

In languages like Java and C#, the keyword “abstract” defines abstract classes. Abstract classes are valuable tools for building hierarchies of related classes that share common characteristics and behaviors while allowing for customization and specialization in derived classes.

What is Interface?

An interface is a fundamental concept in object-oriented programming (OOP) that defines a contract or set of method signatures that must be implemented by any class that adheres to the interface. Interfaces provide a way to achieve multiple inheritance in languages that do not support it directly, and they promote code abstraction, modularity, and flexibility.

Key features and characteristics of interfaces include:

1. Method Signatures: An interface defines a collection of method signatures without implementation details. These method signatures represent a set of behaviors that implementing classes must provide.
2. Contractual Obligation: When a class implements an interface, it must provide concrete implementations for all the methods defined in that interface. Failure to do so results in a compilation error.
3. Multiple Inheritance: Unlike classes, which support single inheritance (extending one class), a class can implement multiple interfaces. This allows a class to inherit and provide multiple sets of behaviors.
4. Abstraction and Polymorphism: Interfaces promote abstraction by separating the “what” (method signatures) from the “how” (method implementations). This abstraction allows for polymorphism, where objects of different classes implementing the same interface can be treated interchangeably.
5. Code Reusability: Interfaces facilitate code reusability by defining common behaviors that various classes can implement. This promotes the DRY (Don’t Repeat Yourself) principle.
6. Flexibility: Interfaces enable loosely coupled code by allowing classes to be interchangeable based on the interfaces they implement. This flexibility is particularly useful in designing modular and extensible systems.

Interfaces are widely used in languages like Java, C#, and TypeScript to define and enforce contracts between classes, ensuring that they adhere to a specified API (Application Programming Interface) and share common behaviors while allowing for diverse implementations.

Comparison Table Between Abstract Class and Interface

Main Differences Between Abstract Class and Interface

Abstract Class:

• Can have both abstract (unimplemented) and concrete (implemented) methods.
• Supports single inheritance, meaning a class can extend only one abstract class.
• Can have constructors to initialize object states.
• Allows for the creation of a base class with default behavior.
• Provides a balance between abstraction and common functionality.
• Useful when creating hierarchies of related classes with shared attributes and behaviors.

Interface:

• Contains only method signatures without any implementations; all methods are implicitly abstract.
• Supports multiple inheritance, meaning a class can implement multiple interfaces.
• Cannot have constructors, as interfaces cannot be instantiated.
• Defines a contract specifying a set of methods that implementing classes must provide.
• Emphasizes abstraction and specifying a shared set of behaviors.
• Useful for defining contracts and ensuring classes adhere to specific API requirements.

Conclusion

An abstract class allows you to create functionality that subclasses can implement or override, whereas an interface allows you to state but not implement functionality. While a class can only extend one abstract class, it can implement several interfaces.

Reference

1. https://dl.acm.org/doi/abs/10.1145/1040305.1040314
2. https://books.google.com/books?hl=en&lr=&id=8M3F_sSSvWkC&oi=fnd&pg=PR13&dq=+interface+java&ots=Qo15NiH18i&sig=Y6OESYd5a6G709ynnLGB4Ry97yU