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

Feature	Abstract Class	Interface
Purpose	Achieves partial implementation	Achieves complete abstraction
Method Implementations	Can have both abstract and concrete methods	All methods are abstract (except default/static methods in Java 8+)
Multiple Inheritance	A class can extend only one abstract class	A class can implement multiple interfaces
Fields/Variables	Can have instance variables of any type	All variables are implicitly public, static, and final
Constructors	Can have constructors	Cannot have constructors
Inheritance Support	Can implement multiple interfaces	Cannot extend an abstract class
Use Case	Useful for sharing common code and state	Useful for defining a contract across unrelated classes

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:

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.
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.
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.
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.

Happy Coding!

Abstraction: Abstract Class vs Interface