🎁Decorator Pattern – Overview

🧠 Concept

The Decorator Pattern lets you add new behavior to objects dynamically without changing their original code or using inheritance.

💡 Think of it as “wrapping” an object with extra features — like adding gift wrap or toppings on a pizza.

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🧱 Key Roles

Role	Description
Component	The base interface or abstract class (defines the main behavior).
ConcreteComponent	The original object to be decorated.
Decorator	Wraps a component and implements the same interface.
ConcreteDecorator	Adds extra behavior before or after delegating to the wrapped component.
Client	Works with components and decorators interchangeably.

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💻 Java Example – UI Example (TextField with Scrollbars & Borders)

// ----- Component -----
interface UIComponent {
    void render();
}

// ----- Concrete Component -----
class TextField implements UIComponent {
    public void render() {
        System.out.println("Rendering basic text field");
    }
}

// ----- Base Decorator -----
abstract class UIComponentDecorator implements UIComponent {
    protected UIComponent component;

    public UIComponentDecorator(UIComponent component) {
        this.component = component;
    }

    public void render() {
        component.render(); // delegate to wrapped component
    }
}

// ----- Concrete Decorators -----
class BorderDecorator extends UIComponentDecorator {
    public BorderDecorator(UIComponent component) {
        super(component);
    }

    public void render() {
        super.render();
        System.out.println("→ Adding border decoration");
    }
}

class ScrollbarDecorator extends UIComponentDecorator {
    public ScrollbarDecorator(UIComponent component) {
        super(component);
    }

    public void render() {
        super.render();
        System.out.println("→ Adding scrollbar decoration");
    }
}

// ----- Client -----
public class Main {
    public static void main(String[] args) {
        // Start with a plain text field
        UIComponent textField = new TextField();

        // Decorate it with scrollbar
        UIComponent scrollable = new ScrollbarDecorator(textField);

        // Further decorate with border
        UIComponent borderedScrollable = new BorderDecorator(scrollable);

        borderedScrollable.render();
    }
}

Output:

Rendering basic text field
→ Adding scrollbar decoration
→ Adding border decoration

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🧠 Flow Summary

Step	Action	Result
1️⃣	Start with TextField	Basic component
2️⃣	Wrap with ScrollbarDecorator	Adds scrolling
3️⃣	Wrap with BorderDecorator	Adds border
✅	Final render	Combines all layers dynamically

🧩 You can add or remove decorators at runtime — flexible layering of behavior.

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🪜 Summary

• Adds functionality without modifying existing classes.

• Promotes composition over inheritance.

• Each decorator is interchangeable with the original component.

• Commonly used for UI elements, streams, and logging in Java.

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🧾 Real-world Analogy

☕ Coffee Order System

• Base = Plain coffee

• Decorators = Milk, Sugar, Caramel, Whipped Cream

• You “wrap” the coffee with each topping → new flavor each time.

💡 The coffee object doesn’t change — decorators just add layers.

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🧭 TL;DR

Decorator Pattern = Add behavior dynamically by wrapping objects. “Flexible alternative to subclassing for extending functionality.”