Java Abstract Class VS Interface Differences

Introduction

In object-oriented programming (OOPs), abstraction is a fundamental concept that allows us to hide the complex implementation details and show only the functionality to the users. In Java, we achieve abstraction using abstract classes and interfaces. However, the fundamental difference between Java abstract class and interfaces in Java lies in their usage and implementation approach. This article provides a beginner-friendly comparison of the difference between an abstract class and interface in Java, illustrated with concrete examples and code snippets.

Abstract Class

An abstract class in Java is a class that cannot be instantiated, meaning you cannot create an object of an abstract class. It can have abstract methods (methods without a body) as well as concrete methods (regular methods with an implementation).

Example:

abstract class Animal {
    abstract void makeSound();

    public void eat() {
        System.out.println("The animal eats");
    }
}

In this example, Animal is an abstract class with an abstract method makeSound() and a concrete method eat().

Use Case:

If you want to create a Dog class that extends Animal, you would need to provide an implementation for makeSound().

class Dog extends Animal {
    void makeSound() {
        System.out.println("The dog barks");
    }
}

class Main {
    public static void main(String[] args) {
        Dog d = new Dog();
        d.makeSound();
        d.eat();
    }
}

In this case, Dog is a subclass of Animal and provides its own implementation of the makeSound() method.

Interface

An interface in Java is a completely abstract type that only contains abstract methods (though since Java 8, interfaces can contain default and static methods). An interface cannot contain a constructor and cannot be instantiated. Classes “implement” an interface to adhere to its defined contract.

Example:

interface Animal {
    void makeSound();
}

class Dog implements Animal {
    public void makeSound() {
        System.out.println("The dog barks");
    }
}

In this example, Animal is an interface with an abstract method makeSound(). The Dog class implements this interface and provides its own implementation of the method.

Differences Between Abstract Class And Interface

Aspect	Abstract Class	Interface
Definition	An abstract class is a class that cannot be instantiated and serves as a blueprint for other classes.	An interface is a contract that defines a set of methods that a class must implement.
Method Implementation	Can provide both implemented and unimplemented methods.	Can only provide method signatures (no method bodies).
Inheritance	Can be inherited by a subclass using the extends keyword.	Can be implemented by a class using the implements keyword.
Multiple Inheritance	Does not support multiple inheritance (a class can only extend one abstract class).	Supports multiple inheritance (a class can implement multiple interfaces).
Fields	Can have instance variables, static variables, and constants.	Can only have constants (static final variables).
Constructor	Can have constructors, which are invoked when creating an instance of the subclass.	Cannot have constructors.
Usage	Often used when there is a common implementation shared by multiple related classes.	Often used when unrelated classes need to implement a common set of methods.
Extension	Can extend a regular class or another abstract class.	Cannot extend a regular class; can only implement interfaces.
Flexibility	Provides more flexibility in terms of adding new methods or modifying existing ones in the future.	Provides less flexibility as changes to the interface can affect all implementing classes.
Partial Implementation	Allows partial implementation of methods (some methods can be implemented while others are left abstract).	Requires all methods to be implemented by the implementing class.

Conclusion

Understanding the differences between abstract class and interface is a crucial part of mastering Java and object-oriented programming. Abstract classes are a good choice when we want to provide common implemented functionality for all subclasses, and interfaces work well when we want to define a role for other classes to implement.

Remember, in Java, the choice between interfaces and abstract classes will depend on the design requirement. Both are powerful tools for achieving abstraction and are vital for writing flexible and maintainable code.