Clean Architecture & Design Patterns in Practice · Lesson

Adapter and Decorator Patterns

Understand how Adapter makes incompatible interfaces work together and Decorator adds responsibilities dynamically.

Adapter and Decorator Patterns is a free Clean Architecture & Design Patterns in Practice lesson on CoddyKit — lesson 1 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the Clean Architecture & Design Patterns in Practice learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.

Structural Patterns: Connect & Enhance

Welcome to Structural Design Patterns! These patterns help you compose objects and classes into larger structures, making systems more flexible and efficient.

In this lesson, we'll explore two powerful patterns:

Adapter Pattern: For making incompatible interfaces work together.
Decorator Pattern: For adding new functionalities to objects dynamically.

The Incompatibility Challenge

Imagine you have an existing system that expects objects to behave in a certain way (a specific interface). Now, you need to integrate a new component that does similar work but has a different interface.

How do you make them talk without changing the existing system or the new component? This is a common challenge in software development.

Meet the Adapter Pattern

The Adapter Pattern acts as a bridge between two incompatible interfaces. It converts the interface of one class into another interface that clients expect.

Think of a universal power adapter: it lets your device (client) plug into any wall socket (adaptee) by converting the physical connection. The device doesn't care about the socket type, only that it gets power.

Roles in the Adapter Pattern

The Adapter Pattern involves three key roles:

Target: The interface the client expects and uses.
Adaptee: The existing class with the incompatible interface that needs adapting.
Adapter: The class that implements the Target interface and wraps an Adaptee instance, translating client requests into calls to the Adaptee.

It allows clients to work with different classes through a single, consistent interface.

Adapter Code Example

Let's see how an adapter can make a modern console writer work with an old logging system.

Our LegacyLogger expects a logMessage method, but our ModernConsoleWriter has writeToConsole.

The ConsoleWriterAdapter bridges this gap.

interface LegacyLogger {
  void logMessage(String message);
}

class ModernConsoleWriter {
  public void writeToConsole(String text) {
    System.out.println("Console: " + text);
  }
}

class ConsoleWriterAdapter implements LegacyLogger {
  private ModernConsoleWriter adaptee;

  public ConsoleWriterAdapter(ModernConsoleWriter adaptee) {
    this.adaptee = adaptee;
  }

  @Override
  public void logMessage(String message) {
    adaptee.writeToConsole("[LEGACY] " + message);
  }
}

public class Main {
  public static void main(String[] args) {
    ModernConsoleWriter writer = new ModernConsoleWriter();
    LegacyLogger logger = new ConsoleWriterAdapter(writer);

    System.out.println("Using the adapter:");
    logger.logMessage("This is an old log entry.");

    System.out.println("\nUsing the original writer directly:");
    writer.writeToConsole("This is a modern console output.");
  }
}

When to Apply Adapter

Consider using the Adapter Pattern when:

You want to use an existing class, but its interface doesn't match the one you need.
You want to create a reusable class that cooperates with unrelated or unforeseen classes, meaning classes that don't necessarily have compatible interfaces.
You're integrating a new component or library into an existing system with a fixed API.

Decorator: Adding Features Dynamically

Sometimes you need to add new responsibilities or behaviors to an object without altering its original structure or affecting other objects of the same class. This is where the Decorator Pattern shines.

It allows you to wrap objects with new functionality, like adding toppings to a base coffee. Each topping (decorator) adds a new cost and description without changing the coffee itself.

Roles in the Decorator Pattern

The Decorator Pattern involves:

Component: The base interface for objects that can be decorated. Both concrete components and decorators implement this.
Concrete Component: The basic object to which responsibilities can be added.
Decorator: An abstract class (or interface) that maintains a reference to a Component object and implements the Component interface.
Concrete Decorators: Specific decorators that add new functionalities to the component.

Decorator Code Example

Let's see how to add milk and sugar to a simple coffee using decorators. Each decorator wraps the existing coffee object and adds its own logic.

This allows for flexible combinations of enhancements.

interface Coffee {
  double getCost();
  String getDescription();
}

class SimpleCoffee implements Coffee {
  @Override
  public double getCost() {
    return 2.0;
  }

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

abstract class CoffeeDecorator implements Coffee {
  protected Coffee decoratedCoffee;

  public CoffeeDecorator(Coffee coffee) {
    this.decoratedCoffee = coffee;
  }

  @Override
  public double getCost() {
    return decoratedCoffee.getCost();
  }

  @Override
  public String getDescription() {
    return decoratedCoffee.getDescription();
  }
}

class MilkDecorator extends CoffeeDecorator {
  public MilkDecorator(Coffee coffee) {
    super(coffee);
  }

  @Override
  public double getCost() {
    return super.getCost() + 0.5;
  }

  @Override
  public String getDescription() {
    return super.getDescription() + ", Milk";
  }
}

class SugarDecorator extends CoffeeDecorator {
  public SugarDecorator(Coffee coffee) {
    super(coffee);
  }

  @Override
  public double getCost() {
    return super.getCost() + 0.2;
  }

  @Override
  public String getDescription() {
    return super.getDescription() + ", Sugar";
  }
}

public class Main {
  public static void main(String[] args) {
    Coffee myCoffee = new SimpleCoffee();
    System.out.println(myCoffee.getDescription() + " $" + myCoffee.getCost());

    myCoffee = new MilkDecorator(myCoffee);
    System.out.println(myCoffee.getDescription() + " $" + myCoffee.getCost());

    myCoffee = new SugarDecorator(myCoffee);
    System.out.println(myCoffee.getDescription() + " $" + myCoffee.getCost());

    Coffee anotherCoffee = new SugarDecorator(new MilkDecorator(new SimpleCoffee()));
    System.out.println(anotherCoffee.getDescription() + " $" + anotherCoffee.getCost());
  }
}

When to Apply Decorator

Use the Decorator Pattern when:

You need to add responsibilities to individual objects dynamically and transparently, without affecting other objects.
You want to extend an object's functionality without using subclassing, which can lead to a "subclass explosion" for many combinations.
You need to allow for flexible combinations of responsibilities.

Quick Check: Pattern Purpose

Read the descriptions below. Which one correctly identifies the primary purpose of the Adapter Pattern?

Recap: Adapter & Decorator

Great job! You've now learned two powerful structural design patterns:

Adapter Pattern: Solves the problem of incompatible interfaces, allowing existing components to work together seamlessly. It's about 'making things fit'.
Decorator Pattern: Solves the problem of adding new responsibilities to objects dynamically, without altering their core structure. It's about 'enhancing functionality'.

These patterns provide flexible ways to structure your code, making it more maintainable and extensible. Keep practicing them!

Frequently asked questions

Is the “Adapter and Decorator Patterns” lesson free?

Yes — the full text of “Adapter and Decorator Patterns” is free to read here on the web, and the Clean Architecture & Design Patterns in Practice course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the Clean Architecture & Design Patterns in Practice course, upgrade to CoddyKit PRO.

What will I learn in “Adapter and Decorator Patterns”?

Understand how Adapter makes incompatible interfaces work together and Decorator adds responsibilities dynamically. You practise Clean Architecture & Design Patterns in Practice with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start Clean Architecture & Design Patterns in Practice?

No prior experience is required. Clean Architecture & Design Patterns in Practice on CoddyKit is structured for beginners through advanced learners; this is — lesson 1 of 4, so you can start here or from the beginning and move at your own pace.

How long does the “Adapter and Decorator Patterns” lesson take?

Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.

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