When to use an interface instead of an abstract class and vice versa?

Understanding when to use an interface versus an abstract class is essential for creating flexible, maintainable, and efficient object-oriented designs. Both are used to define methods and properties for classes to implement, but their purposes and use cases differ. Let's break it down.

Think of Real-World Example

Imagine you're designing a transportation system:

1. An interface is like saying, "Anything that can transport people should have a start() and stop() method," whether it's a car, plane, or bike. It defines a contract, but it doesn't care how these are implemented.
2. An abstract class is like saying, "All motor vehicles have some shared features like an engine, so let’s provide those features but leave specifics (like fuel type) for individual vehicles like cars or trucks to decide." It provides a base but leaves gaps to fill.

When to Use an Interface

1. Define a Contract for Unrelated Classes

Use interfaces when you want multiple unrelated classes to follow the same contract or set of rules.
Example: A Flyable interface could be implemented by both a Bird and a Plane.

from abc import ABC, abstractmethod

class Flyable(ABC):
    @abstractmethod
    def fly(self):
        pass

class Bird(Flyable):
    def fly(self):
        return "Flapping wings"

class Plane(Flyable):
    def fly(self):
        return "Engaging jet engines"

2. Enable Multiple Inheritance

In languages like Java, a class can implement multiple interfaces but only extend one class. This makes interfaces useful when you need to combine behaviors from different contracts.
Example: A Smartphone might implement both Camera and GPS interfaces.

3. Promote Decoupling

Interfaces define what a class should do without tying it to a specific implementation. This promotes flexibility and testability.

When to Use an Abstract Class

1. Share Common Behavior

Use an abstract class when multiple related classes share some functionality, but each subclass also needs to implement specific details.
Example: A Shape abstract class might have a draw() method (to be implemented) and a shared calculate_area() method.

from abc import ABC, abstractmethod

class Shape(ABC):
    def __init__(self, color):
        self.color = color

    @abstractmethod
    def draw(self):
        pass

    def get_color(self):
        return self.color

class Circle(Shape):
    def draw(self):
        return "Drawing a Circle"

class Square(Shape):
    def draw(self):
        return "Drawing a Square"

2. Provide Partial Implementation

An abstract class can include default behavior or properties that all subclasses inherit but can override if needed. Interfaces can't do this.

3. Represent a Strong IS-A Relationship

Abstract classes are better when the relationship is strong, like a Dog IS-A Mammal. Use them when creating a base class for closely related objects.

Key Differences Between Interfaces and Abstract Classes

When to Use Each

1. Use an Interface When:

   • You need to define a contract without dictating implementation.
   • Multiple classes, potentially unrelated, need to follow the same rules.
   • Multiple inheritance of behaviors is required.

2. Use an Abstract Class When:

   • You need to share common behavior or state among related classes.
   • You want to provide some default functionality.
   • A strong IS-A relationship exists between the base and derived classes.

Summary

To decide between an interface and an abstract class:

• Interfaces focus on the "what" (contract). Use them when you need flexibility and multiple behaviors.
• Abstract classes focus on the "how" (shared implementation). Use them when you have related classes sharing a base structure or behavior.

For more insights into designing flexible, scalable systems, explore Grokking System Design Fundamentals or Grokking the System Design Interview on DesignGurus.io. These resources will help you master design patterns and object-oriented principles.