TL;Dr

Both Proxy and Decorator patterns wrap objects to modify behavior, but they serve different purposes. Decorators add new features (like adding toppings to a pizza), while Proxies control access or behavior (like a security guard at a building). Decorators can be stacked together, Proxies typically can’t. Use Decorator when you want to enhance functionality, use Proxy when you need to control or manage access.

What Are Design Patterns Anyway? 🤔

Before we dive in, let’s quickly understand what design patterns are. Think of them as tried-and-tested recipes for solving common programming problems. Just like how there’s a standard way to make a sandwich, there are standard ways to structure your code to solve specific problems.

Real-World Analogies First! 🌍

Decorator Pattern = Pizza Toppings 🍕

Imagine you’re at a pizza place:

• You start with a basic pizza (your original object)
• You add pepperoni (+$2) - that’s one decorator
• You add extra cheese (+$1.50) - another decorator
• You add mushrooms (+$1) - yet another decorator

Each topping adds to your pizza without changing what a pizza fundamentally is. You can keep stacking toppings!

Proxy Pattern = Security Guard 👮

Think of a proxy like a security guard at a fancy building: The guard checks if you have permission to enter They might log who goes in and out They control access to the actual building You can’t stack multiple guards - one is enough!

Similarities

Both patterns wrap an object to modify how we interact with it:

1. They don’t change the interface - Both patterns implement the same interface as the object they’re wrapping
2. They introduce a middle layer - There’s always an intermediary between the client and the real object
3. They’re structural patterns - Both patterns deal with how objects are composed

  graph LR
    Client --> Wrapper
    Wrapper --> RealObject

Key Differences 🔍

1. Required vs. Optional Dependency

Decorator NEEDS an object to decorate:

// You MUST pass a file to decorate
FileDecorator decorator = new CompressionDecorator(actualFile);
// Without actualFile, the decorator is useless!

Proxy can exist without the real object:

// Proxy can create the expensive object later (lazy loading)
ImageProxy proxy = new ImageProxy("huge-image.jpg");
// The actual image isn't loaded until you really need it
proxy.display(); // NOW it loads the image

2. Additive vs. Controlling

Decorators ADD functionality:

// Original file behavior + compression
CompressedFile = new CompressionDecorator(file);
// Original behavior is preserved, just enhanced

// Like adding whipped cream to coffee - still coffee, just better!

Proxies CONTROL access or behavior:

// Proxy might deny access entirely
SecureFile = new SecurityProxy(file);
secureFile.read(); // Might throw "Access Denied!"

// Like a bouncer at a club - you might not get in at all!

  graph LR
    Client --> SecurityProxy
    SecurityProxy -->|Authorized| RealService
    SecurityProxy -->|Unauthorized| X[Access Denied]

3. Class Chaining

Decorators are meant to be stacked:

// Start with basic file
File myFile = new File("document.txt");

// Add compression
File compressedFile = new CompressionDecorator(myFile);

// Add encryption on top of compression
File secureCompressedFile = new EncryptionDecorator(compressedFile);

// Add logging on top of everything
File fullyDecoratedFile = new LoggingDecorator(secureCompressedFile);

  graph LR
    Client --> LoggingDecorator
    LoggingDecorator --> EncryptionDecorator
    EncryptionDecorator --> CompressionDecorator
    CompressionDecorator --> File

Proxies typically work alone:

// Just one proxy is enough
File secureFile = new SecurityProxy(new File("secret.txt"));

// Multiple proxies? Usually not needed and can get confusing
// (Though technically possible, it's rarely done)

When to Use Which? 🤷

Use Decorator When:

• You want to add responsibilities to objects dynamically
• You need to add multiple functionalities that can be combined
• You want to extend functionality without modifying existing code
• Example: Adding filters to images, adding features to text editors

Use Proxy When:

• You need to control access to an object
• You want to add lazy initialization (load expensive resources only when needed)
• You need to log access or add security checks
• You want to hide the complexity of the real object
• Example: Database connections, network requests, access control

Common Use Cases in Real Apps 💻

Decorator in the Wild:

• Java I/O Streams: BufferedInputStream(FileInputStream)
• UI Components: Adding scroll bars, borders to windows
• Web Development: Middleware in Express.js

Proxy in the Wild:

• Spring Framework: AOP proxies for transactions
• Hibernate: Lazy loading of entities
• JavaScript: Proxy objects for reactive frameworks

Conclusion 🎯

While Proxy and Decorator patterns might look similar at first glance (both wrap objects), they serve very different purposes:

• Decorator = Enhancement 🎨 (Make it better)
• Proxy = Control 🚦 (Manage access/behavior)

Remember: Decorators are like Instagram filters - you can stack many to enhance your photo. Proxies are like security systems - one good system is usually enough!

Practice Exercise 🏋️

Try implementing both patterns for a simple DataSource interface:

1. Create decorators for compression and encryption
2. Create a proxy for caching frequently accessed data
3. Notice how decorators stack but the proxy stands alone

Happy coding! 🚀

References

https://doeken.org/blog/decorator-vs-proxy-pattern https://refactoring.guru/design-patterns/decorator/java/example https://www.youtube.com/@ChristopherOkhravi