When to use which structural pattern in LLD - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Choosing the right structural pattern for flexible UI components

You are designing a UI library where components can be combined in different ways. Which structural pattern is best when you want to treat individual components and groups of components uniformly?

A Facade pattern

B Composite pattern

C Adapter pattern

D Decorator pattern

💡 Hint

Think about a pattern that lets you work with single objects and collections the same way.

The Composite pattern allows you to treat individual objects and compositions of objects uniformly, which is ideal for flexible UI components.

❓ Architecture

intermediate

Using Facade pattern for simplifying complex subsystems

You have a complex system with many classes and interfaces. You want to provide a simple interface to clients without exposing the complexity. Which structural pattern fits best?

A Facade pattern

B Proxy pattern

C Bridge pattern

D Flyweight pattern

💡 Hint

Look for a pattern that provides a simplified interface to a complex system.

The Facade pattern provides a unified and simple interface to a complex subsystem, hiding its complexity from clients.

❓ scaling

advanced

Scaling object creation with Flyweight pattern

You need to create millions of similar objects in a memory-efficient way. Which structural pattern helps reduce memory usage by sharing common parts?

A Adapter pattern

B Decorator pattern

C Flyweight pattern

D Bridge pattern

💡 Hint

Think about sharing intrinsic state to save memory.

The Flyweight pattern shares common parts of objects to reduce memory usage when many similar objects are needed.

❓ tradeoff

advanced

Tradeoffs between Adapter and Bridge patterns

You want to decouple an abstraction from its implementation so they can vary independently. Which pattern is more suitable, and what is a key difference from the other?

A Bridge; it separates abstraction and implementation, while Adapter converts one interface to another

B Bridge; it converts one interface to another, while Adapter separates abstraction and implementation

C Adapter; it shares objects to save memory, while Bridge provides a simple interface

D Bridge; it shares objects to save memory, while Adapter provides a simple interface

💡 Hint

Focus on the purpose of each pattern regarding interfaces and abstraction.

Bridge separates abstraction from implementation to allow them to vary independently. Adapter changes one interface to another to make incompatible interfaces work together.

❓ component

expert

Designing a plugin system with Decorator pattern

You are building a plugin system where features can be added dynamically to objects without changing their code. Which structural pattern allows wrapping objects to add responsibilities at runtime?

A Flyweight pattern

B Facade pattern

C Composite pattern

D Decorator pattern

💡 Hint

Consider a pattern that wraps objects to add new behavior dynamically.

The Decorator pattern wraps objects to add responsibilities dynamically without modifying the original object code, ideal for plugin systems.

Practice

1. Which structural pattern is best suited for representing part-whole hierarchies like folders and files in a system?

easy

A. Facade pattern

B. Composite pattern

C. Proxy pattern

D. Decorator pattern

Solution

1. Step 1: Understand the problem context

   Part-whole hierarchies like folders and files form tree structures where each node can be a composite or a leaf.

2. Step 2: Match the pattern to the structure

   The Composite pattern is designed to treat individual objects and compositions uniformly, perfect for tree-like structures.

3. Final Answer:

   Composite pattern -> Option B

4. Quick Check:

   Tree structures = Composite pattern [OK]

Hint: Tree or hierarchy? Use Composite pattern [OK]

Common Mistakes:

• Confusing Decorator with Composite
• Using Proxy for hierarchy management
• Choosing Facade for internal structure

2. Which pattern should you use if you want to add responsibilities to objects dynamically without changing their class?

easy

A. Proxy pattern

B. Facade pattern

C. Composite pattern

D. Decorator pattern

Solution

1. Step 1: Identify the need to add features dynamically

   When you want to add new behavior to objects at runtime without altering their class, you need a flexible pattern.

2. Step 2: Select the pattern that wraps objects

   The Decorator pattern wraps objects to add responsibilities dynamically, without changing the original class.

3. Final Answer:

   Decorator pattern -> Option D

4. Quick Check:

   Add features dynamically = Decorator [OK]

Hint: Add features on the fly? Use Decorator [OK]

Common Mistakes:

• Choosing Proxy instead of Decorator
• Using Facade to add features
• Confusing Composite with Decorator

3. Consider a system where you want to control access to a resource and add lazy loading. Which structural pattern fits best?

class ResourceProxy {
  constructor() {
    this.resource = null;
  }
  access() {
    if (!this.resource) {
      this.resource = new RealResource();
    }
    return this.resource;
  }
}

medium

A. Proxy pattern

B. Decorator pattern

C. Composite pattern

D. Facade pattern

Solution

1. Step 1: Analyze the code behavior

   The code delays creating the real resource until it is accessed, controlling access and adding lazy loading.

2. Step 2: Identify the matching pattern

   This behavior matches the Proxy pattern, which controls access to another object and can add lazy initialization.

3. Final Answer:

   Proxy pattern -> Option A

4. Quick Check:

   Control access + lazy load = Proxy [OK]

Hint: Control access or lazy load? Use Proxy [OK]

Common Mistakes:

• Confusing Proxy with Decorator
• Using Facade for access control
• Choosing Composite for resource control

4. You have a complex subsystem with many classes. You want to provide a simple interface to clients. Which pattern should you use? A developer wrote this code but clients still see complex calls:

class ComplexSystem {
  operationA() {}
  operationB() {}
  operationC() {}
}

class SimpleInterface {
  constructor() {
    this.system = new ComplexSystem();
  }
  simpleOperation() {
    this.system.operationA();
    this.system.operationB();
    this.system.operationC();
  }
}

What is the likely mistake?

medium

A. They did not hide the ComplexSystem instance properly

B. They used Proxy instead of Facade

C. They used Composite instead of Facade

D. They forgot to add Decorator to SimpleInterface

Solution

1. Step 1: Understand Facade pattern purpose

   Facade provides a simple interface hiding complex subsystem details from clients.

2. Step 2: Identify the mistake in code usage

   If clients still see complex calls, likely the ComplexSystem instance is exposed or clients bypass the Facade.

3. Final Answer:

   They did not hide the ComplexSystem instance properly -> Option A

4. Quick Check:

   Facade hides complexity = hide subsystem [OK]

Hint: Facade hides complexity; don't expose subsystem [OK]

Common Mistakes:

• Confusing Proxy with Facade
• Exposing subsystem objects to clients
• Using Decorator to simplify interface

5. You are designing a graphic editor. You want to represent shapes that can be simple (circle, square) or groups of shapes. You also want to add features like border color dynamically and control access to some shapes. Which combination of structural patterns should you use?

hard

A. Facade for groups, Proxy for features, Composite for access control

B. Decorator for groups, Composite for features, Facade for access control

C. Composite for groups, Decorator for features, Proxy for access control

D. Proxy for groups, Facade for features, Decorator for access control

Solution

1. Step 1: Match patterns to requirements

   Groups of shapes form tree structures best handled by Composite. Adding features dynamically fits Decorator. Controlling access fits Proxy.

2. Step 2: Combine patterns correctly

   Use Composite for shape groups, Decorator to add border color dynamically, and Proxy to control access to shapes.

3. Final Answer:

   Composite for groups, Decorator for features, Proxy for access control -> Option C

4. Quick Check:

   Groups=Composite, Features=Decorator, Access=Proxy [OK]

Hint: Groups=Composite, Features=Decorator, Access=Proxy [OK]

Common Mistakes:

• Mixing Facade with group representation
• Using Proxy for adding features
• Confusing Decorator with access control