DEV Community

Dev Cookies
Dev Cookies

Posted on

📝 Day Four: Decorator Design Pattern in Java

🔍 What is the Decorator Pattern?

The Decorator Pattern is a structural design pattern that allows behavior to be added to individual objects, either statically or dynamically, without altering the structure of the original class.

✅ When Should You Use It?

  • When you want to add responsibilities to objects dynamically at runtime.
  • When subclassing would lead to too many classes.
  • To follow Open-Closed Principle: classes should be open for extension but closed for modification.

🧠 Real-World Analogy

Imagine you order a coffee ☕. You start with a base coffee, and then decorate it with milk, sugar, mocha, or whipped cream — all dynamically layered, without modifying the base coffee class.

🧱 Structure 

 +-------------------+ | Component | <-- Interface +-------------------+ ^ | +------------------------+ | ConcreteComponent | <-- Base Implementation +------------------------+ ^ | +------------------------+ | Decorator | <-- Abstract Wrapper +------------------------+ ^ ------------------------ | | | MilkDecorator SugarDecorator etc. <-- Concrete Decorators
Enter fullscreen mode Exit fullscreen mode

☕ Example: Coffee Shop

✅ 1. Component Interface 

public interface Coffee { String getDescription(); double cost(); }
Enter fullscreen mode Exit fullscreen mode

✅ 2. ConcreteComponent 

public class SimpleCoffee implements Coffee { @Override public String getDescription() { return "Simple Coffee"; } @Override public double cost() { return 5.0; } }
Enter fullscreen mode Exit fullscreen mode

✅ 3. Abstract Decorator 

public abstract class CoffeeDecorator implements Coffee { protected Coffee decoratedCoffee; public CoffeeDecorator(Coffee coffee) { this.decoratedCoffee = coffee; } public String getDescription() { return decoratedCoffee.getDescription(); } public double cost() { return decoratedCoffee.cost(); } }
Enter fullscreen mode Exit fullscreen mode

✅ 4. Concrete Decorators 

public class MilkDecorator extends CoffeeDecorator { public MilkDecorator(Coffee coffee) { super(coffee); } public String getDescription() { return super.getDescription() + ", Milk"; } public double cost() { return super.cost() + 1.5; } } public class SugarDecorator extends CoffeeDecorator { public SugarDecorator(Coffee coffee) { super(coffee); } public String getDescription() { return super.getDescription() + ", Sugar"; } public double cost() { return super.cost() + 0.5; } } public class MochaDecorator extends CoffeeDecorator { public MochaDecorator(Coffee coffee) { super(coffee); } public String getDescription() { return super.getDescription() + ", Mocha"; } public double cost() { return super.cost() + 2.0; } }
Enter fullscreen mode Exit fullscreen mode

💻 Client Code 

public class DecoratorDemo { public static void main(String[] args) { Coffee coffee = new SimpleCoffee(); System.out.println(coffee.getDescription() + " $" + coffee.cost()); coffee = new MilkDecorator(coffee); coffee = new SugarDecorator(coffee); coffee = new MochaDecorator(coffee); System.out.println(coffee.getDescription() + " $" + coffee.cost()); } }
Enter fullscreen mode Exit fullscreen mode

🧪 Output 

Simple Coffee $5.0 Simple Coffee, Milk, Sugar, Mocha $9.0
Enter fullscreen mode Exit fullscreen mode

🎯 Benefits

✅ Follows Open-Closed Principle

✅ Runtime behavior change

✅ Composable and reusable wrappers

✅ Cleaner than multiple subclasses

🧑‍🏫 Key Takeaways

  • The Decorator Pattern is perfect when you want to extend behavior without modifying classes.
  • It helps you avoid creating too many subclasses.
  • You can keep adding layers of decorators to wrap original functionality.

🧩 Java Libraries Using Decorator

  • BufferedReader, BufferedInputStream, PrintWriter in java.io 🎯 
BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));
Enter fullscreen mode Exit fullscreen mode

🚀 Up Next for Day 5: Want to go with Strategy, Adapter, State, or Chain of Responsibility?

Let me know your pick and I’ll deliver another deep-dive, code-rich, real-world blog for the next one!

Top comments (0)