Dev Cookies

Posted on Apr 12

📝 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

☕ Example: Coffee Shop

✅ 1. Component Interface

public interface Coffee {
    String getDescription();
    double cost();
}

✅ 2. ConcreteComponent

public class SimpleCoffee implements Coffee {
    @Override
    public String getDescription() {
        return "Simple Coffee";
    }

    @Override
    public double cost() {
        return 5.0;
    }
}

✅ 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();
    }
}

✅ 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;
    }
}

💻 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());
    }
}

🧪 Output

Simple Coffee $5.0
Simple Coffee, Milk, Sugar, Mocha $9.0

🎯 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));

🚀 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!

DEV Community