Interface or an Abstract Class: which one to use?

The choice between using an interface and an abstract class depends on the specific requirements of your design. Both define contracts for other classes, but they serve different purposes and have unique use cases. Here's how to decide:

When to Use an Interface

1. Define a Contract for Unrelated Classes

   • Use interfaces when you want to specify a set of behaviors that can be implemented by unrelated classes.
   • Example: Flyable can be implemented by both a Bird and a Drone, even though they are not related by inheritance.

   interface Flyable {
       void fly();
   }
   
   class Bird implements Flyable {
       @Override
       public void fly() {
           System.out.println("Flapping wings");
       }
   }
   
   class Drone implements Flyable {
       @Override
       public void fly() {
           System.out.println("Engaging propellers");
       }
   }

2. Multiple Inheritance of Behavior

   • In languages like Java, a class can implement multiple interfaces but can inherit from only one class. Use interfaces to combine behaviors.
   • Example: A Smartphone can implement both Camera and GPS.

3. Flexibility and Decoupling

   • Interfaces promote loose coupling. A class implementing an interface focuses on the contract rather than the specific implementation details.

4. Stateless and Behavior-Only Requirements

   • Use interfaces when you only need to define behavior without requiring shared state or common functionality.

When to Use an Abstract Class

1. Provide a Base for Related Classes

   • Use an abstract class when creating a base class for closely related classes that share common functionality.
   • Example: Animal can serve as a base class for Dog and Cat.

   abstract class Animal {
       String name;
   
       Animal(String name) {
           this.name = name;
       }
   
       abstract void makeSound();
   
       void eat() {
           System.out.println(name + " is eating");
       }
   }
   
   class Dog extends Animal {
       Dog(String name) {
           super(name);
       }
   
       @Override
       void makeSound() {
           System.out.println("Bark!");
       }
   }

2. Share Common Implementation

   • Abstract classes can provide partial implementation (concrete methods) that subclasses can use or override.

3. Encapsulation of State and Behavior

   • If you need to maintain common fields or properties (e.g., name in the Animal class), use an abstract class.

4. Strong IS-A Relationship

   • Abstract classes are better for defining a strong hierarchical relationship, such as Vehicle being a base class for Car and Bike.

5. Default Implementation

   • Abstract classes allow you to define default behaviors that subclasses inherit unless overridden.

Key Differences

Summary of When to Use

1. Use an Interface When:

   • You need multiple inheritance of behaviors.
   • Unrelated classes need to implement the same behavior.
   • You want to define a purely behavioral contract.

2. Use an Abstract Class When:

   • You have shared fields or concrete methods.
   • Classes share a strong hierarchical relationship.
   • You want to provide a base implementation for subclasses.

For robust software design, combine both when needed: use interfaces for broad contracts and abstract classes for shared functionality within a hierarchy. To master object-oriented programming and design, explore Grokking System Design Fundamentals or Grokking Advanced Coding Patterns for Interviews on DesignGurus.io! These resources delve into such design principles in depth.