Why Decorator pattern in LLD? - Purpose & Use Cases

The Big Idea

What if you could add new features anytime without rewriting your whole code?

The Scenario

Imagine you have a simple coffee order system. You want to add options like milk, sugar, or whipped cream. Without a smart design, you might create a new class for every combination, like CoffeeWithMilk, CoffeeWithSugar, CoffeeWithMilkAndSugar, and so on.

The Problem

This manual way quickly becomes a mess. You end up with many classes, hard to maintain and extend. Adding a new option means creating even more classes. It's slow, confusing, and error-prone.

The Solution

The Decorator pattern lets you add features dynamically by wrapping objects. Instead of many classes, you wrap your coffee with decorators like MilkDecorator or SugarDecorator. This keeps code simple, flexible, and easy to extend.

Before vs After

✗ Before

class CoffeeWithMilkAndSugar extends Coffee { ... }

✓ After

coffee = new SugarDecorator(new MilkDecorator(new SimpleCoffee()))

What It Enables

It enables flexible, reusable, and easy-to-maintain code by adding features without changing existing code.

Real Life Example

Think of ordering a burger where you add toppings one by one. Each topping wraps the burger, adding flavor without changing the original burger.

Key Takeaways

Manual class explosion is hard to manage.

Decorator pattern wraps objects to add features dynamically.

It keeps code clean, flexible, and easy to extend.

Practice

1. What is the main purpose of the Decorator pattern in system design?

easy

A. To create multiple instances of a class

B. To add new features to objects without changing their original code

C. To replace an object with a completely new one

D. To store objects in a database

Solution

1. Step 1: Understand the Decorator pattern goal

   The Decorator pattern is designed to add new behavior or features to objects dynamically without modifying their original code.

2. Step 2: Compare options with the pattern goal

   Options B, C, and D do not describe adding features without changing code. Only To add new features to objects without changing their original code matches the pattern's purpose.

3. Final Answer:

   To add new features to objects without changing their original code -> Option B

4. Quick Check:

   Decorator pattern = add features without code change [OK]

Hint: Decorator adds features by wrapping, not by changing code [OK]

Common Mistakes:

• Confusing Decorator with Factory pattern
• Thinking it replaces objects instead of wrapping
• Assuming it stores data instead of adding behavior

2. Which of the following is the correct way to implement a decorator in a class-based design?

easy

A. Use a global variable to store extra features

B. Modify the original class to include new methods directly

C. Delete the original class and write a new one with extra features

D. Create a new class that wraps the original class and adds new methods

Solution

1. Step 1: Recall Decorator implementation style

   Decorator pattern uses a wrapper class that holds an instance of the original class and adds new behavior.

2. Step 2: Evaluate each option

   Create a new class that wraps the original class and adds new methods correctly describes wrapping with a new class. Options A, B, and C do not follow the decorator pattern principles.

3. Final Answer:

   Create a new class that wraps the original class and adds new methods -> Option D

4. Quick Check:

   Decorator = wrapper class adding methods [OK]

Hint: Decorator wraps original class; never modifies it directly [OK]

Common Mistakes:

• Changing original class instead of wrapping
• Replacing class instead of extending behavior
• Using global variables for decoration

3. Consider this pseudocode using the Decorator pattern:

class Coffee:
    def cost(self):
        return 5

class MilkDecorator:
    def __init__(self, coffee):
        self.coffee = coffee
    def cost(self):
        return self.coffee.cost() + 2

order = MilkDecorator(Coffee())
print(order.cost())

What will be the output of print(order.cost())?

medium

A. 7

B. 5

C. 2

D. Error

Solution

1. Step 1: Understand the base cost

   The Coffee class returns 5 when cost() is called.

2. Step 2: Calculate decorated cost

   The MilkDecorator adds 2 to the coffee cost by calling self.coffee.cost() and adding 2.

3. Final Answer:

   7 -> Option A

4. Quick Check:

   5 + 2 = 7 [OK]

Hint: Decorator adds to base cost by calling wrapped object's method [OK]

Common Mistakes:

• Forgetting to call wrapped object's method
• Returning only decorator's added cost
• Confusing base and added costs

4. In the following code snippet, what is the main issue that prevents the decorator from working correctly?

class LoggerDecorator:
    def __init__(self, component):
        self.component = component
    def execute(self):
        print('Logging start')
        self.execute()
        print('Logging end')

medium

A. The decorator calls itself recursively causing infinite loop

B. The decorator does not call the wrapped component's execute method

C. The constructor is missing a parameter

D. The print statements are incorrect

Solution

1. Step 1: Analyze method calls inside execute()

   The execute() method calls self.execute(), which calls itself recursively without a base case.

2. Step 2: Identify correct call to wrapped component

   The decorator should call self.component.execute() to delegate the call, not call itself.

3. Final Answer:

   The decorator calls itself recursively causing infinite loop -> Option A

4. Quick Check:

   Recursive call without base case = infinite loop [OK]

Hint: Decorator must call wrapped object's method, not itself [OK]

Common Mistakes:

• Calling self.execute() instead of wrapped component
• Missing delegation to wrapped object
• Ignoring recursion issues

5. You are designing a notification system where messages can be sent via Email, SMS, and Push notifications. You want to add features like encryption and logging dynamically without changing the core notification classes. How would you apply the Decorator pattern here?

hard

A. Use inheritance to create subclasses for every combination of features

B. Add encryption and logging code directly inside each notification class

C. Create decorator classes for encryption and logging that wrap notification objects

D. Store all notifications in a database and process features later

Solution

1. Step 1: Identify need for dynamic feature addition

   Encryption and logging should be added without modifying core notification classes, so dynamic wrapping is needed.

2. Step 2: Apply Decorator pattern for flexible feature addition

   Create decorator classes that wrap notification objects and add encryption or logging behavior dynamically.

3. Final Answer:

   Create decorator classes for encryption and logging that wrap notification objects -> Option C

4. Quick Check:

   Decorator pattern = dynamic wrapping for features [OK]

Hint: Wrap notifications with decorators for features, avoid code changes [OK]

Common Mistakes:

• Modifying core classes directly
• Using inheritance explosion for feature combos
• Ignoring dynamic behavior needs