Structural patterns (Adapter, Decorator, Facade) in Software Engineering - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Understanding the Adapter Pattern

Which of the following best describes the main purpose of the Adapter pattern?

A It adds new responsibilities to an object dynamically without changing its interface.

B It creates objects without specifying the exact class of object to create.

C It allows incompatible interfaces to work together by converting one interface into another.

D It provides a simplified interface to a complex subsystem.

💡 Hint

Think about how two different plugs can fit into the same socket using a converter.

The Adapter pattern acts like a bridge between two incompatible interfaces, allowing them to work together without changing their existing code.

🧠 Conceptual

intermediate

Role of the Decorator Pattern

What is the primary benefit of using the Decorator pattern in software design?

A It converts one interface into another to enable compatibility.

B It allows behavior to be added to individual objects dynamically without affecting others.

C It simplifies a complex system by hiding its details behind a single interface.

D It ensures only one instance of a class exists throughout the application.

💡 Hint

Consider how you might add toppings to a pizza one by one without changing the pizza itself.

The Decorator pattern lets you add new features to objects at runtime, keeping the original object unchanged and allowing flexible extensions.

🚀 Application

advanced

Identifying the Facade Pattern Use Case

You have a complex library with many classes and methods. You want to provide a simple interface for common tasks so users don't get overwhelmed. Which pattern should you apply?

A Facade

B Decorator

C Adapter

D Singleton

💡 Hint

Think about how a TV remote simplifies controlling many devices.

The Facade pattern offers a simple interface to a complex subsystem, making it easier for users to interact with it without understanding all details.

🔍 Analysis

advanced

Comparing Adapter and Decorator Patterns

Which statement correctly distinguishes the Adapter pattern from the Decorator pattern?

A Both Adapter and Decorator simplify complex systems by hiding details behind a single interface.

B Decorator ensures only one instance exists, Adapter creates new objects dynamically.

C Adapter adds new behavior dynamically, while Decorator converts one interface to another.

D Adapter changes an object's interface to match another, while Decorator adds new behavior without changing the interface.

💡 Hint

Focus on whether the pattern changes the interface or adds behavior.

Adapter changes the interface so two incompatible interfaces can work together. Decorator adds new behavior to an object without changing its interface.

❓ Reasoning

expert

Choosing the Right Structural Pattern

A software system has multiple complex subsystems. You want to provide a simple interface for clients, allow adding new features to objects dynamically, and enable incompatible interfaces to work together. Which combination of patterns best achieves these goals?

A Facade for simple interface, Decorator for adding features, Adapter for interface compatibility

B Adapter for simple interface, Facade for adding features, Decorator for interface compatibility

C Decorator for simple interface, Adapter for adding features, Facade for interface compatibility

D Singleton for simple interface, Adapter for adding features, Facade for interface compatibility

💡 Hint

Match each pattern to its main strength: Facade simplifies, Decorator extends, Adapter connects.

The Facade pattern simplifies complex subsystems with a simple interface. The Decorator pattern adds new features dynamically. The Adapter pattern enables incompatible interfaces to work together.

Practice

1. Which structural pattern allows incompatible interfaces to work together by converting one interface into another?

easy

A. Decorator

B. Adapter

C. Facade

D. Singleton

Solution

1. Step 1: Understand the role of Adapter pattern

   The Adapter pattern converts the interface of one class into another interface clients expect, enabling incompatible interfaces to work together.

2. Step 2: Compare with other patterns

   Decorator adds features, Facade simplifies interfaces, Singleton controls instance creation. Only Adapter changes interfaces for compatibility.

3. Final Answer:

   Adapter -> Option B

4. Quick Check:

   Interface conversion = Adapter [OK]

Hint: Adapter = interface converter between incompatible parts [OK]

Common Mistakes:

• Confusing Decorator with Adapter
• Thinking Facade changes interfaces
• Mixing Singleton with structural patterns

2. Which of the following is the correct way to describe the Decorator pattern?

easy

A. It converts one interface to another.

B. It provides a simple interface to a complex system.

C. It adds new responsibilities to objects dynamically.

D. It ensures a class has only one instance.

Solution

1. Step 1: Identify the purpose of Decorator pattern

   Decorator adds new features or responsibilities to objects dynamically without changing their structure.

2. Step 2: Eliminate other options

   It provides a simple interface to a complex system. describes Facade, It converts one interface to another. describes Adapter, It ensures a class has only one instance. describes Singleton pattern.

3. Final Answer:

   It adds new responsibilities to objects dynamically. -> Option C

4. Quick Check:

   Decorator = add features dynamically [OK]

Hint: Decorator = add features without changing original object [OK]

Common Mistakes:

• Mixing Decorator with Facade
• Confusing Adapter with Decorator
• Thinking Singleton is a structural pattern

3. Consider a system where a Facade pattern is used to simplify multiple complex subsystems. What is the main benefit when a client uses the Facade?

medium

A. The client interacts with a simple interface hiding complexity.

B. The client must adapt its interface to each subsystem.

C. The client can access all subsystem details directly.

D. The client can add new features to subsystems dynamically.

Solution

1. Step 1: Understand Facade pattern role

   Facade provides a simple unified interface to a set of interfaces in a subsystem, hiding complexity from the client.

2. Step 2: Analyze options

   The client can access all subsystem details directly. is wrong because Facade hides details. The client must adapt its interface to each subsystem. describes Adapter. The client can add new features to subsystems dynamically. describes Decorator.

3. Final Answer:

   The client interacts with a simple interface hiding complexity. -> Option A

4. Quick Check:

   Facade = simple interface to complex system [OK]

Hint: Facade hides complexity behind a simple interface [OK]

Common Mistakes:

• Thinking Facade exposes all subsystem details
• Confusing Facade with Adapter or Decorator
• Assuming Facade adds features dynamically

4. A developer tries to use an Adapter pattern but the client code still fails due to interface mismatch. What is the most likely error?

medium

A. The client code is using the correct interface.

B. The Decorator pattern was used instead of Adapter.

C. The Facade pattern was applied to simplify the interface.

D. The Adapter does not implement the target interface correctly.

Solution

1. Step 1: Identify Adapter pattern requirement

   Adapter must implement the target interface expected by the client to convert calls properly.

2. Step 2: Analyze failure cause

   If the client still fails, likely the Adapter does not correctly implement or map the target interface methods.

3. Final Answer:

   The Adapter does not implement the target interface correctly. -> Option D

4. Quick Check:

   Adapter interface mismatch = implementation error [OK]

Hint: Adapter must match target interface exactly [OK]

Common Mistakes:

• Confusing Decorator or Facade with Adapter errors
• Assuming client code is always correct
• Ignoring interface method signatures

5. You have a legacy payment system with a complex API. You want to provide a simple interface for new clients and also add logging features without changing the legacy code. Which combination of structural patterns should you use?

hard

A. Facade to simplify interface and Decorator to add logging

B. Decorator to simplify interface and Adapter to add logging

C. Adapter to add logging and Facade to simplify interface

D. Adapter to simplify interface and Decorator to add logging

Solution

1. Step 1: Identify pattern for simplifying complex API

   Facade provides a simple interface to a complex system, ideal for the legacy payment system.

2. Step 2: Identify pattern for adding features without changing code

   Decorator adds new responsibilities like logging dynamically without modifying original code.

3. Step 3: Combine patterns logically

   Use Facade to simplify the interface for clients, then wrap with Decorator to add logging features.

4. Final Answer:

   Facade to simplify interface and Decorator to add logging -> Option A

5. Quick Check:

   Simplify = Facade, Add features = Decorator [OK]

Hint: Facade simplifies, Decorator adds features without code change [OK]

Common Mistakes:

• Using Adapter to simplify interface instead of Facade
• Confusing roles of Adapter and Decorator
• Trying to add logging via Adapter