Difference between Abstract Class and Interface

In object-oriented programming, understanding the difference between abstract class and interface is vital for efficient software design. Both concepts define reusable structures, yet they serve distinct purposes. 

Abstract classes are ideal for shared behavior in closely related objects, while interfaces enforce contracts across unrelated classes. Choosing between them can impact scalability, maintainability, and performance. 

This guide explores abstract class vs interface, breaking down definitions, key distinctions, similarities, and use cases. You’ll also find examples, including abstract class vs interface in Java, to clarify their real-world applications. Let’s get started.

What Are Interfaces and How Do They Work?

Interfaces are a blueprint for a class in programming. They define a set of methods that a class must implement without dictating how the methods should be performed.

Understanding how interfaces work is crucial for grasping the difference between abstract class and interface. Below is a clear explanation of how they function.

• Interfaces Act as Contracts: An interface specifies methods that a class must implement. For example, if a Vehicle interface includes a startEngine() method, every class implementing this interface, like Car or Bike, must define the functionality of startEngine().
• Interfaces Support Multiple Inheritance: A class can implement multiple interfaces, allowing you to achieve multiple inheritance in languages like Java. For example, a class FlyingCar can implement both Vehicle and Flyable interfaces.
• Interfaces Ensure Consistency: They enforce a uniform structure across classes. This makes them especially useful in larger projects where standardization is important.

Key characteristics of interfaces are equally significant in understanding abstract class vs interface. The following points highlight their distinct features.

• Only Method Declarations Allowed: Interfaces can only declare methods, not define them. For instance, the Drawable interface can declare draw() but not implement it.
• Fields in Interfaces are Static and Final: Variables in an interface, like int MAX_SPEED = 120;, are constants. They cannot be changed once assigned.
• Cannot Instantiate Interfaces: You cannot create objects of an interface. Instead, a class must implement it to provide concrete behavior.

Understanding syntax is key to mastering the difference between abstract class and interface. Below is an example of how to declare an interface.

interface Animal {
    void makeSound(); // Method declaration
}

class Dog implements Animal {
    public void makeSound() {
        System.out.println("Bark");
    }
}

public class Main {
    public static void main(String[] args) {
        Animal dog = new Dog();
        dog.makeSound();
    }
}

Notice how the Animal interface defines the makeSound() method, and the Dog class provides the implementation. This example also illustrates how interfaces enforce consistency.

Up next, you will explore the unique traits of abstract classes, deepening your understanding of abstract class vs interface.

What Are Abstract Classes and How Are They Used?

Abstract classes are special classes in object-oriented programming that cannot be instantiated on their own. They serve as a blueprint for other classes, containing abstract methods (without implementation) and non-abstract methods (with implementation). 

Abstract classes are defined using the abstract keyword and are essential for designing reusable and hierarchical code structures.

Below are key ways abstract classes are used. Each point highlights practical scenarios to help you understand their importance.

• Define Common Behavior: Abstract classes provide shared functionality for subclasses. For instance, an abstract class Shape can define a method calculatePerimeter() while leaving calculateArea() as abstract for each shape to implement.
• Facilitate Code Reusability: Abstract classes allow subclasses to inherit and reuse common methods. For example, a Vehicle class can define startEngine() once for all subclasses like Car or Bike.
• Support Polymorphism: Abstract classes enable polymorphism by serving as a base type. For example, you can treat all subclasses of Animal uniformly using Animal as a reference type.

Key characteristics of abstract classes further define their role in programming. Below are their standout features.

• Can Have Both Abstract and Non-Abstract Methods: Abstract classes can mix abstract methods (e.g., abstract void displayInfo();) and concrete methods (e.g., void printDetails()).
• Allow Field Declarations: Abstract classes can have instance variables, unlike interfaces. For example, protected String name; in an abstract class allows storing common data.
• Cannot Be Instantiated Directly: You cannot create objects of an abstract class. Instead, a subclass must implement the abstract methods to make them usable.
• Can Have Constructors: Abstract classes can include constructors to initialize common fields or execute shared logic. For example, public AbstractClass(String name) sets a default name for all subclasses.

Here’s an example to illustrate abstract classes in action:

abstract class Animal {
    String name;

    Animal(String name) {
        this.name = name;
    }

    abstract void makeSound();

    void sleep() {
        System.out.println(name + " is sleeping.");
    }
}

class Dog extends Animal {
    Dog(String name) {
        super(name);
    }

    void makeSound() {
        System.out.println(name + " says: Bark");
    }
}

public class Main {
    public static void main(String[] args) {
        Animal dog = new Dog("Buddy");
        dog.makeSound();
        dog.sleep();
    }
}

Output:

Buddy says: Bark
Buddy is sleeping.

This example demonstrates how an abstract class (Animal) can define both abstract (makeSound) and concrete (sleep) methods while allowing subclasses (Dog) to implement specific behaviors.

Next, delve into the difference between abstract class and interface. This comparison will clarify their unique roles and help you choose the right one. 

Difference Between Abstract Class and Interface: Key Insights

Understanding the difference between abstract class and interface is essential for choosing the right tool for your programming needs.

Below is a comprehensive comparison between abstract class vs interface. These key parameters will clarify their unique roles and applications.

Also Read: Abstract Class in Java and Methods [With Examples]

The table above highlights the primary difference between abstract class and interface, giving you a clear picture of their unique attributes.

Up next, dive into the similarities between abstract class vs interface in Java. Understanding their common ground will further sharpen your conceptual clarity.

Abstract Class vs Interface: Similarities Explained

Abstract class vs interface in Java may seem different, but they share certain traits that make them indispensable in object-oriented programming.

Below are the common similarities between abstract class and interface. These points will help you connect their overlapping features.

• Used to Define a Contract: Both are tools to define a structure for other classes. For example, both can declare methods that must be implemented in derived classes.
• Cannot Be Instantiated: You cannot create objects of either. A PaymentGateway interface implemented by PayPal and Stripe or an abstract Vehicle class for Car and Bike cannot be directly instantiated. Attempting Vehicle vehicle = new Vehicle(); or PaymentGateway gateway = new PaymentGateway(); will throw an error.
• Encourage Abstraction: Both promote abstraction by hiding implementation details and exposing only the essential functionalities.
• Serve as Base Types: Abstract classes and interfaces can both be used as reference types. For example, a variable of an interface type (Animal animal = new Dog();) can refer to any implementing class.
• Support Polymorphism: Both enable polymorphism by allowing different implementations to be treated uniformly. For instance, you can call methods on objects of different classes that share the same interface or abstract class.
• Play a Key Role in Large Systems: Both are vital for designing modular and scalable systems. They help ensure that code adheres to a consistent structure, which is especially useful in teams or long-term projects.

Build a strong foundation in Java programming with upGrad's free core Java basics course. Master essential concepts and improve your coding skills for advanced topics.

Next, learn how to decide when to use an abstract class vs interface based on practical scenarios and project needs.

Abstract Class vs Interface: How to Choose the Right One

Knowing when to use an abstract class vs interface ensures your code is efficient, maintainable, and suited to the task.

Below are key considerations to guide your decision. Each point highlights specific scenarios with examples to clarify their practical usage.

• Use Abstract Class for Shared Behavior: Choose an abstract class when multiple derived classes share common functionality. For example, an abstract class Animal can have a breathe() method implemented once and shared by Dog and Cat classes.
• Use Interface for Defining Contracts: Opt for an interface when unrelated classes need to follow the same structure. For instance, Flyable can be implemented by both Bird and Airplane classes to ensure both have a fly() method.
• Use Abstract Class for Partial Implementation: If you need some methods to have a default implementation, use an abstract class. For example, Shape can have an implemented method calculatePerimeter() while keeping calculateArea() abstract.
• Use Interface for Achieving Multiple Inheritance: An interface is better when a class must inherit behavior from multiple sources. For example, a class FlyingCar can implement both Vehicle and Flyable interfaces.
• Use Abstract Class for Closely Related Classes: Abstract classes work best when classes are part of a tightly related hierarchy. For instance, Vehicle can serve as a base class for Car and Truck, sharing attributes like numberOfWheels.
• Use Interface for Plug-and-Play Behavior: Interfaces are ideal when you need to add interchangeable functionality. For example, an interface PaymentMethod can have implementations like CreditCard and PayPal.

Similar Read: Runnable Interface in Java: Implementation, Steps & Errors

Choosing between abstract class vs interface in Java depends on understanding the project's needs and the role each plays in your design.

Next, explore how abstract class vs interface can work together to build robust systems in complex scenarios.

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Abstract Class vs Interface: Integrating Them in Large Systems

Integrating abstract class vs interface effectively can streamline large systems by ensuring modularity, scalability, and maintainability. Understanding their combined usage is critical for tackling complex design challenges.

Below are examples of how abstract class and interface can complement each other in large-scale applications. These scenarios demonstrate their practical integration.

• Use Abstract Class for Core Structure and Interface for Add-Ons: Define a foundational abstract class for shared features and use interfaces for optional behaviors. For example, an abstract class Vehicle can have shared attributes like numberOfWheels, while interfaces like Flyable or Sailable can add specific functionalities.
• Combine for Flexibility and Consistency: Use interfaces to enforce contracts and abstract classes to provide common methods. For instance, an interface Worker can define work() and takeBreak() methods, while an abstract class Employee can implement shared logic like calculatePay().
• Abstract Classes for Hierarchical Relationships, Interfaces for Cross-Cutting Concerns: Use abstractions for logical hierarchies (e.g., Account -> SavingsAccount or CurrentAccount) and interfaces for concerns like Auditable or Printable that apply across unrelated classes.
• Achieve Multiple Inheritance with Interfaces and Shared Behavior with Abstract Classes: A class can inherit behavior from an abstract class and implement multiple interfaces. For example, a SmartPhone class can extend an abstract class Device while implementing interfaces like Camera and MusicPlayer.
• Enhance Testing and Mocking: Interfaces make unit testing easier by allowing mocks. Combine them with abstract classes for reusable base test logic. For example, an abstract class DatabaseTest can include common setup steps, while interfaces like DatabaseConnection enable flexible testing across database types.

Also Read: Abstraction in Java

Abstract class vs interface in Java, when used together, provides the best of both worlds—shared functionality through abstract classes and flexibility through interfaces.

Next, learn how upGrad can guide you in mastering abstract class and interface concepts for real-world applications.

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