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Interfaces vs Abstract Classes in Java

Feature	Interface	Abstract Class
Purpose	Defines a contract for classes to follow	Provides a partial implementation with optional method bodies
Keyword	interface	abstract
Inheritance Type	Multiple inheritance is allowed	Only single inheritance
Methods	All methods are abstract by default (Java 7); Java 8+ supports default and static methods	Can have abstract and concrete (implemented) methods
Variables	public static final (constants) only	Can have instance variables with any access modifier
Constructors	No constructors	Yes, can have constructors
Access Modifiers	Methods are public by default	Methods/fields can be private, protected, public, etc.
Implements vs Extends	A class implements an interface	A class extends an abstract class
Use Case	Define a common behavior without implementation	Provide shared code and enforce method implementation

Example: Interface

    
    interface Vehicle {
        void start();  // abstract method
    }

    class Car implements Vehicle {
        public void start() {
            System.out.println("Car started");
        }
    }
    
    

Example: Abstract Class

    
    abstract class Animal {
        abstract void makeSound();  // abstract method

        void sleep() {
            System.out.println("Animal is sleeping");
        }
    }

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

When to Use What?

Scenario	Use
You need to define common behavior but no implementation	Use Interface
You want to share some code and enforce method implementation	Use Abstract Class
You want to achieve multiple inheritance	Use Interfaces
You want to model real-world hierarchy	Use Abstract Class (e.g., Animal → Dog)

Java 8+ Enhancements

Interfaces can now have:

• default methods (with body)
• static methods

"Interface = “Tell me what to do, I’ll decide how.”"
"Abstract Class = “Here’s how to do some of it, but you must finish the rest.”"

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