When to use which structural pattern in LLD - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to select the structural pattern used to simplify complex object creation.

LLD

Use the [1] pattern to build complex objects step by step.

Drag options to blanks, or click blank then click option'

ADecorator

BBuilder

CAdapter

DFacade

Attempts:

3 left

💡 Hint

Common Mistakes

Confusing Builder with Adapter or Decorator patterns.

The Builder pattern helps construct complex objects step by step, making it easier to create different representations.

2fill in blank

medium

Complete the code to choose the pattern that allows incompatible interfaces to work together.

LLD

The [1] pattern helps convert one interface to another expected by clients.

Drag options to blanks, or click blank then click option'

AAdapter

BFlyweight

CProxy

DComposite

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing Adapter with Proxy or Composite patterns.

The Adapter pattern acts as a bridge between incompatible interfaces, allowing them to work together.

3fill in blank

hard

Fix the error in the code to select the pattern that provides a simplified interface to a complex system.

LLD

Use the [1] pattern to hide complex subsystem details behind a simple interface.

Drag options to blanks, or click blank then click option'

ABridge

BMediator

CFacade

DDecorator

Attempts:

3 left

💡 Hint

Common Mistakes

Confusing Facade with Decorator or Mediator patterns.

The Facade pattern offers a simple interface to a complex system, making it easier for clients to use.

4fill in blank

hard

Fill both blanks to select the pattern that composes objects into tree structures and lets clients treat them uniformly.

LLD

The [1] pattern allows clients to treat individual objects and compositions as [2].

Drag options to blanks, or click blank then click option'

AComposite

BFlyweight

CComponent

DProxy

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing Composite with Flyweight or Proxy patterns.

The Composite pattern composes objects into tree structures. The 'Component' is the common interface for both leaf and composite objects.

5fill in blank

hard

Fill all three blanks to select the pattern that adds responsibilities to objects dynamically and the correct terms for the base and added objects.

LLD

The [1] pattern attaches additional [2] to an object dynamically. The base object implements the [3] interface.

Drag options to blanks, or click blank then click option'

ADecorator

Bbehaviors

CComponent

DAdapter

Attempts:

3 left

💡 Hint

Common Mistakes

Confusing Decorator with Adapter or Proxy patterns.

The Decorator pattern adds behaviors to objects dynamically. Both the base object and decorators implement the Component interface.

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