Abstraction: Abstract Class vs Interface

When designing software in Java, choosing between abstract classes and interfaces can have a big impact on flexibility, maintainability, and readability. In this post, we’ll explore their key differences, when to use one over the other, and look at practical examples to help you master this concept.

Key Differences Between Abstract Class and Interface

When to Use Abstract Class vs Interface?

Use Abstract Class When:

• You need to share state or behavior across related classes.
  
  Example: Employee can have fields like name and id with a common getDetails() method, shared among its subclasses Manager and Lead.
  

  abstract class Employee {
      String name;
      int id;
  
      Employee(String name, int id) {
          this.name = name;
          this.id = id;
      }
  
      // Concrete method shared among subclasses
      public void getDetails() {
          System.out.println(name + " (ID: " + id + ")");
      }
  
      // Abstract method to be implemented by subclasses
      abstract double getSalary();
  }
  
  class Manager extends Employee {
      Manager(String name, int id) {
          super(name, id);
      }
  
      @Override
      double getSalary() {
          return 75000;
      }
  }
  

• You want to define fields and constructors that subclasses can use.

• You need to provide partial implementations or utility methods for subclasses.

Use Interface When:

• You want to define a common behavior across unrelated classes.
  
  Example: Both Car and Drone could implement an Operable interface, as they both share a start() method but are unrelated classes.
  

  interface Operable {
      void start();
      void stop();
  }
  
  class Car implements Operable {
      @Override
      public void start() {
          System.out.println("Car started.");
      }
  
      @Override
      public void stop() {
          System.out.println("Car stopped.");
      }
  }
  
  class Drone implements Operable {
      @Override
      public void start() {
          System.out.println("Drone started.");
      }
  
      @Override
      public void stop() {
          System.out.println("Drone stopped.");
      }
  }
  

• You need multiple inheritance to combine different behaviors. For example, a class can implement both Runnable and Serializable.

• You want to define default methods to add new functionality without breaking backward compatibility.

Interview Insights

Common Interview Questions:

1. Why Can’t an Interface Have Constructors?
   
   Since interfaces define pure abstractions, there’s no need for constructors. Only abstract classes, which contain some implementation, require constructors to initialize state.

2. Why Use Abstract Classes Instead of Interfaces?
   
   Use abstract classes when:

   • You need to share code among related classes.
   • You want to define state (fields) and constructors.

3. Can an Abstract Class Implement an Interface?
   
   Yes! Abstract classes can implement one or more interfaces. They can even provide default implementations for the interface methods.

4. Can You Use Both Abstract Classes and Interfaces in the Same Class?
   
   Yes, a class can extend an abstract class and implement multiple interfaces. This allows for flexible design patterns and multiple inheritance.

Summary

• Abstract classes are ideal when you need partial implementation and shared state across related classes. They allow for common fields, constructors, and utility methods.
• Interfaces are best suited for complete abstraction, multiple inheritance, and defining common behavior for unrelated classes. They allow you to evolve your codebase with default methods while maintaining backward compatibility.

By mastering the differences and knowing when to use each, you’ll be better equipped to design scalable, maintainable software systems.

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Happy Coding!