Decorator pattern in LLD - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Purpose of the Decorator Pattern

Which of the following best describes the main purpose of the Decorator pattern in system design?

A To add responsibilities to objects dynamically without changing their interface

B To create a fixed set of subclasses for different behaviors

C To replace inheritance with composition for all cases

D To enforce a single instance of a class throughout the system

💡 Hint

Think about how you can add features to an object without altering its core code.

The Decorator pattern allows adding new behavior to objects dynamically by wrapping them, without modifying their interface or using inheritance.

❓ Architecture

intermediate

Component Structure in Decorator Pattern

In the Decorator pattern, which component is responsible for defining the interface that both the core object and decorators implement?

A Concrete Component

B Decorator

C Component Interface

D Client

💡 Hint

This component ensures that decorators and core objects can be used interchangeably.

The Component Interface defines the common interface for both the core object and decorators, allowing decorators to wrap the core object transparently.

❓ scaling

advanced

Scaling Decorators in a High-Traffic System

When applying multiple decorators to enhance an object in a high-traffic system, what is a key consideration to maintain scalability?

A Avoid using decorators and duplicate code instead

B Use inheritance instead of decorators for better performance

C Apply decorators only at compile time to avoid runtime costs

D Minimize the number of decorator layers to reduce overhead

💡 Hint

Think about how wrapping many layers can affect system speed.

Each decorator adds a layer of wrapping which can increase processing time; minimizing layers helps keep the system responsive under load.

❓ tradeoff

advanced

Tradeoff of Using Decorator Pattern

What is a common tradeoff when using the Decorator pattern extensively in a system?

A Loss of object-oriented principles by avoiding inheritance

B Increased complexity in debugging due to many small wrapper objects

C Reduced flexibility because decorators cannot be combined

D Decreased code reuse because decorators duplicate functionality

💡 Hint

Consider how many layers of wrapping affect understanding the flow.

Using many decorators can make the system harder to debug and understand because behavior is spread across multiple small objects.

❓ component

expert

Request Flow in Decorator Pattern

Given a core object wrapped by two decorators, which sequence correctly describes the flow of a method call from the client?

A Client calls outer decorator → outer decorator calls inner decorator → inner decorator calls core object

B Client calls inner decorator → inner decorator calls outer decorator → outer decorator calls core object

C Client calls core object → core object calls inner decorator → inner decorator calls outer decorator

D Client calls outer decorator → core object → inner decorator

💡 Hint

Think about how wrappers delegate calls inward.

The client interacts with the outermost decorator, which delegates the call to the next inner decorator, and finally to the core object.

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