How to implement the Decorator design pattern in Java?
Jul 13, 2025 am 02:29 AM
How to implement the decorator design pattern in Java? You can extend object functions by defining component interfaces, creating specific components, building basic decorators, adding specific decorators, and using decorators to combine them. 1. Define the component interface Coffee, including cost() and description() methods; 2. Create a specific component SimpleCoffee to implement basic functions; 3. Build an abstract decorator class CoffeeDecorator to implement the same interface and hold component references; 4. Add specific decorators such as MilkDecorator and SugarDecorator to add new functions based on the original behavior; 5. Use nested calls to combine decorators to achieve dynamic function extensions, such as new SugarDecorator(new MilkDecorator(new SimpleCoffee()))) at runtime.
Sure, here's how to implement the Decorator design pattern in Java in a practical and clear way.
What is the Decorator Pattern?
The Decorator pattern lets you add behavior or responsibility to objects dynamically, without affecting the behavior of other objects of the same class. It's a flexible alternative to subclassing for extending functionality.
In Java, this pattern is commonly used in I/O streams like InputStream , OutputStream , Reader , and Writer .
When Should You Use It?
You should reach for the Decorator pattern when:
- You want to add features to an object dynamically.
- Subclassing would lead to too many combinations or become unmanageable.
- You need to follow the Open/Closed Principle : open for extension, closed for modification.
This often comes up when dealing with things like formatting, logging, encryption, compression, etc.
How to Implement the Decorator Pattern in Java
Let's walk through a simple example using coffee and toppings (like milk, sugar) to demonstrate how decorators work.
1. Define the Component Interface
Start by defining the base interface that both the concrete component and decorators will implement.
public interface Coffee {
double cost();
String description();
}2. Create the Concrete Component
This is the basic object you'll be decorating.
public class SimpleCoffee implements Coffee {
public double cost() {
return 2.0;
}
public String description() {
return "Simple Coffee";
}
}3. Create the Base Decorator Class
This abstract class holds a reference to the component and implements the same interface.
public abstract class CoffeeDecorator implements Coffee {
protected Coffee decoratedCoffee;
public CoffeeDecorator(Coffee coffee) {
this.decoratedCoffee = coffee;
}
public double cost() {
return decoratedCoffee.cost();
}
public String description() {
return decoratedCoffee.description();
}
}4. Add Concrete Decorators
Each decorator adds its own behavior on top of the existing one.
For example, a milk decorator:
public class MilkDecorator extends CoffeeDecorator {
public MilkDecorator(Coffee coffee) {
super(coffee);
}
public double cost() {
return super.cost() 0.5;
}
public String description() {
return super.description() ", Milk";
}
}And a sugar decorator:
public class SugarDecorator extends CoffeeDecorator {
public SugarDecorator(Coffee coffee) {
super(coffee);
}
public double cost() {
return super.cost() 0.2;
}
public String description() {
return super.description() ", Sugar";
}
}5. Use the Decorators
Now you can combine decorators as needed at runtime:
public class Main {
public static void main(String[] args) {
Coffee coffee = new SimpleCoffee();
System.out.println(coffee.cost()); // Output: 2.0
System.out.println(coffee.description()); // Output: Simple Coffee
Coffee milkCoffee = new MilkDecorator(new SimpleCoffee());
System.out.println(milkCoffee.cost()); // Output: 2.5
System.out.println(milkCoffee.description()); // Output: Simple Coffee, Milk
Coffee milkAndSugarCoffee = new SugarDecorator(new MilkDecorator(new SimpleCoffee()));
System.out.println(milkAndSugarCoffee.cost()); // Output: 2.7
System.out.println(milkAndSugarCoffee.description()); // Output: Simple Coffee, Milk, Sugar
}
}Key Points to Remember
- Each decorator wraps another instance of the same interface.
- The client code doesn't need to know whether it's dealing with a base component or a decorated one.
- Keep each decorator focused on a single responsibility.
- Avoid deep nesting of decorators — it can get hard to read and debug.
If you're working with streams in Java, think about how BufferedInputStream wraps any InputStream to add buffering. That's the exact same idea!
Wrap-Up
Implementing the Decorator pattern in Java isn't complicated once you understand the structure. You define a common interface, create a base implementation, then build reusable wrappers that enhance behavior without modifying existing classes. It's clean, scalable, and fits well into systems where flexibility matters.
That's basically it.
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