What is the purpose of a decorator in programming?

Decorator is used for dynamically adding responsibilities to an object. It wraps the original object and delegates method calls, while adding or modifying behavior before or after the main method call. This allows extending functionality without changing the original class. The main advantage is flexible combination of different decorators.

// Interface defining the basic functionality
interface Component {
    String operation();
}

// Concrete class implementing the interface
class ConcreteComponent implements Component {
    @Override
    public String operation() {
        return "Basic function";
    }
}

// Abstract decorator class
abstract class Decorator implements Component {
    protected Component component;

    public Decorator(Component component) {
        this.component = component;
    }

    @Override
    public String operation() {
        return component.operation();
    }
}

// Concrete decorator adding new behavior
class ConcreteDecoratorA extends Decorator {
    public ConcreteDecoratorA(Component component) {
        super(component);
    }

    @Override
    public String operation() {
        return "Decorator A(" + super.operation() + ")";
    }
}

// Another concrete decorator
class ConcreteDecoratorB extends Decorator {
    public ConcreteDecoratorB(Component component) {
        super(component);
    }

    @Override
    public String operation() {
        return "Decorator B(" + super.operation() + ")";
    }
}

/*
// Usage example:
public class Main {
    public static void main(String[] args) {
        Component simple = new ConcreteComponent();
        System.out.println("Simple object: " + simple.operation());

        Component decoratorA = new ConcreteDecoratorA(simple);
        System.out.println("Object with decorator A: " + decoratorA.operation());

        Component decoratorB = new ConcreteDecoratorB(decoratorA);
        System.out.println("Object with decorators A and B: " + decoratorB.operation());
    }
}
*/

Comparison with inheritance:

Aspect	Decorator	Inheritance
Extension	Dynamic addition of behavior at runtime	Static extension at compile time
Flexibility	Easily combine multiple decorators	Creates rigid class hierarchy
Multiple functions	Each decorator adds one function	Multiple class increases when adding functions
Behavior change	Wraps the object without changing its class	Requires modification of base or derived class