Interface vs. Abstract Class – When to Use Which?

Understanding the Difference

How do you decide when to use an interface and when to use an abstract class? Let’s break it down.

Use Cases for Interfaces

• Multiple Implementations: Use an interface when you expect multiple classes to implement the same set of methods but with different logic. For example, you might have an interface Drivable that is implemented by both Car and Boat, each with its own way of "driving."

• Loose Coupling: Interfaces are ideal when you want to decouple your code. If you define methods in an interface, you can swap out different implementations without changing the code that uses those methods.

Use Cases for Abstract Classes

• Shared Code: Use an abstract class when you have some common behavior (methods) that should be shared across all subclasses, but you also want to enforce certain rules by requiring specific methods to be implemented.

• Single Inheritance: If you know that a class should inherit from only one parent class and that this parent class should provide some base functionality, an abstract class is the way to go.

Performance Considerations

Generally, interfaces might lead to slightly slower performance due to the indirection in method calls (especially before Java 8). However, the difference is usually negligible and shouldn’t be a deciding factor unless in performance-critical situations.

Real-World Example

Imagine you’re designing a system for different types of payment methods:

• Interface Example: Define a Payable interface for different payment methods like CreditCard, PayPal, and Bitcoin. Each implements the processPayment() method differently.

• Abstract Class Example: Suppose you also want to group all online payment methods. You could create an abstract class OnlinePayment that implements a method for connecting to a payment gateway but leaves the specific payment processing method (processPayment()) to be defined by subclasses like CreditCard or PayPal.
  

public interface Payable {
    void processPayment(double amount);
}

public abstract class OnlinePayment implements Payable {
    void connectToGateway() {
        System.out.println("Connecting to online payment gateway...");
    }
}

public class CreditCard extends OnlinePayment {
    public void processPayment(double amount) {
        connectToGateway();
        System.out.println("Processing credit card payment of " + amount + " €");
    }
}

Challenge: Decide and Design

Given a scenario where you have different types of users (Admin, Guest, Registered), think about whether you’d use an interface, an abstract class, or both. Design a simple structure and implement it in Java.

Conclusion

Choosing between interfaces and abstract classes isn’t always straightforward, but understanding the strengths and weaknesses of each can help you make the best decision for your design. Interfaces offer flexibility and the ability to implement multiple behaviors, while abstract classes provide a strong foundation with shared functionality.

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