What are the key differences between an Interface and an Abstract class?

Interface vs Abstract Class

Interface

• 100% abstraction (prior to Java 8).
• Can have default/static methods (Java 8+).
• No constructors.
• Multiple inheritance allowed.

Abstract Class

• Partial abstraction.
• Can have constructors and fields.
• Single inheritance only.

Examples

Interface

interface Animal {
    void makeSound(); // Method declaration
}
class Dog implements Animal {
    public void makeSound() {
        System.out.println("Bark");
    }
}

Abstract Class

abstract class Vehicle {
    abstract void start(); // Abstract method
    void stop() {
        System.out.println("Vehicle stopped"); // Concrete method
    }
}
class Car extends Vehicle {
    void start() {
        System.out.println("Car started");
    }
}

When to Use What?

Use Interface When:

• You need to define a contract for unrelated classes.
• Multiple inheritance of type is required.
• You want to provide default or static methods without affecting the implementing classes.

Use Abstract Class When:

• Classes share a common base and need to share code.
• You want to provide some implemented methods and enforce others to be overridden.
• You need constructors or non-constant fields.

Conclusion

Both interfaces and abstract classes are powerful tools in Java, and choosing between them depends on your application's needs. Use interfaces to define behaviors across unrelated classes and abstract classes for shared code in a class hierarchy.

By understanding their differences and strengths, you can write cleaner and more maintainable code. Happy coding! 🚀