Design Patterns Interview Questions

Creational Patterns

1. What is the Singleton pattern?

• • Answer: Ensures only one instance of a class exists and provides a global point of access to it.
  • Example: Database connection, logging, configuration settings.

2. Explain the Factory Method pattern.

• • Answer: Defines an interface for creating an object, but lets subclasses decide which class to instantiate.  
     
  • Example: Creating different types of documents (PDF, Word, Excel).

3. What is the Abstract Factory pattern?

• • Answer: Provides an interface for creating families of related or dependent objects without specifying their concrete classes.
  • Example: Creating UI elements for different operating systems (Windows, macOS, Linux).  
     

4. Describe the Builder pattern.

• • Answer: Separates the construction of a complex object from its representation, allowing the same construction process to create different representations.   
     
  • Example: Building a car with different options (engine, color, accessories).   
     

5. What is the Prototype pattern?

• • Answer: Creates new objects by copying existing prototypes instead of creating them from scratch.
  • Example: Cloning documents, creating copies of complex objects.

Structural Patterns

6. Explain the Adapter pattern.

• • Answer: Converts the interface of a class into another interface clients expect.
  • Example: Using a third-party library with a different interface.

7. What is the Bridge pattern?

• • Answer: Decouples an abstraction from its implementation so that they can vary independently.
  • Example: Abstraction: Shape (Circle, Square), Implementation: Color (Red, Green, Blue).

8. Describe the Composite pattern.

Answer: Composes objects into tree structures to represent part-whole hierarchies.

• • Example: File system, organizational charts.

9. What is the Decorator pattern?

• • Answer: Dynamically adds responsibilities to an object without altering its class.
  • Example: Adding toppings to a pizza, adding features to a software component.

10. Explain the Facade pattern.

• • Answer: Provides a simplified interface to a complex subsystem.
  • Example: Home theater system, complex library API.

11. What is the Flyweight pattern?

• • Answer: Reduces memory usage by sharing common objects among multiple clients.
  • Example: String interning, rendering many similar graphical objects.

12. Describe the Proxy pattern.

• • Answer: Provides a surrogate or placeholder for another object to control access to it.
  • Example: Remote proxy, virtual proxy, protection proxy.

Behavioral Patterns

13. Explain the Chain of Responsibility pattern.

• • Answer: Passes a request along a chain of handlers until one of them handles it.
  • Example: Request processing in a web server, event handling.

14. What is the Command pattern?

• • Answer: Encapsulates a request as an object, making it possible to parameterize clients with different requests.
  • Example: Undo/redo functionality, macro commands.

15. Describe the Interpreter pattern.

• • Answer: Defines a grammatical representation for a language and an interpreter that uses the representation to interpret sentences in the language.
  • Example: Expression evaluation, compilers.

16. What is the Iterator pattern?

• • Answer: Provides a way to access the elements of an aggregate object sequentially without exposing its underlying representation.  
     
  • Example: Traversing a collection of objects.

17. Explain the Mediator pattern.

• • Answer: Defines an object that encapsulates how a set of objects interact.
  • Example: Chat application, user interface components.

18. What is the Memento pattern?

• • Answer: Captures and externalizes an object’s internal state so that it can be restored to this state later.
  • Example: Undo/redo functionality, game state saving.

19. Describe the Observer pattern.

• • Answer: Defines a one-to-many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically.   
     
  • Example: Event handling, stock market data feeds.

20. What is the State pattern?

• • Answer: Allows an object to alter its behavior when its internal state changes.
  • Example: Finite state machine, traffic light control.

21. Explain the Strategy pattern.

• • Answer: Defines a family of algorithms, encapsulates each one, and makes them interchangeable.
  • Example: Sorting algorithms, search algorithms.

22. What is the Template Method pattern.

• • Answer: Defines the skeleton of an algorithm in an operation, deferring some steps to subclasses.
  • Example: Abstract class for network protocols, framework for web applications.

23. Describe the Visitor pattern.

• • Answer: Represents an operation to be performed on the elements of an object structure.
  • Example: Adding new operations to existing object structures, code generation.

24. When would you use the Singleton pattern, and what are some potential drawbacks?

• When to use: When you need to ensure only one instance of a class exists throughout the application, such as for global configuration, logging, or a database connection pool.
• Drawbacks:
  • Testability: Can make unit testing more difficult due to the global nature of the instance.
  • Flexibility: Can limit flexibility and make it harder to change the implementation later.
  • Violation of SOLID principles: Can violate the Single Responsibility Principle and make the class difficult to maintain.

25. How does the Factory Method pattern differ from the Abstract Factory pattern?

• Factory Method: Creates a single object of a specific type.
• Abstract Factory: Creates a family of related or dependent objects.

26. What are some real-world examples of the Builder pattern?

• Building a car: Configuring different options like engine, color, and accessories.
• Creating a complex document: Specifying sections, formatting, and images.
• Generating HTML pages: Defining the structure, content, and styles.

27. How can the Prototype pattern be used to improve performance?

• By creating copies of existing objects instead of creating new ones from scratch, it can avoid expensive object creation processes, especially for complex objects.

28. Explain the difference between the Adapter and Decorator patterns.

• Adapter: Changes the interface of an existing class to match a different interface.
• Decorator: Adds new functionality to an existing object without changing its interface.

29. When is the Bridge pattern a suitable solution?

• When you need to decouple an abstraction from its implementation so that they can vary independently. For example, when you have a set of abstractions and implementations, and you want to be able to combine them flexibly.

30. How does the Composite pattern simplify tree-like data structures?

• By representing part-whole hierarchies, it allows you to treat individual objects and compositions of objects uniformly.

31. What are some common use cases for the Decorator pattern?

• Adding toppings to a pizza.
• Adding features to a software component (e.g., logging, caching).
• Applying different styles to UI elements.

32. How can the Facade pattern improve code maintainability?

• By providing a simplified interface to a complex subsystem, it reduces the complexity for clients and makes the code easier to understand and modify.

33. Explain the trade-offs of using the Flyweight pattern.

• Benefits: Reduced memory usage.
• Drawbacks: Increased complexity in managing shared objects and potential performance overhead due to the need to check for existing shared objects.

34. What are the different types of Proxy patterns, and how do they differ?

• Remote Proxy: Provides a local representative for an object residing on a different machine.
• Virtual Proxy: Creates a placeholder for a large or expensive object and loads it only when needed.
• Protection Proxy: Controls access to an object based on permissions.

35. How does the Chain of Responsibility pattern improve code flexibility?

• It allows you to add or remove handlers from the chain dynamically, making it easy to modify the behavior of the request processing without affecting other parts of the system.

36. What are some advantages of using the Command pattern?

• Supports undo/redo functionality.
• Allows you to parameterize clients with different requests.
• Makes it easier to log and queue requests.

37. Explain the limitations of the Interpreter pattern.

• Can be complex to implement for complex grammars.
• May not be efficient for all types of languages.

38. How does the Iterator pattern improve code reusability?

• It provides a generic way to traverse different collections without exposing their internal structure.

39. When is the Mediator pattern a good choice for object interaction?

• When you have complex interactions between multiple objects, and you want to reduce the dependencies between them.

40. How can the Memento pattern be used to implement undo/redo functionality?

• By capturing and storing the internal state of an object, it allows you to restore the object to a previous state.

41. What are some real-world examples of the Observer pattern?

• Event handling in user interfaces.
• Stock market data feeds.
• Social media notifications.

42. How does the State pattern help manage complex object behavior?

• By encapsulating the object’s state and the corresponding behavior, it makes it easier to manage and modify the object’s behavior based on its current state.

43. When is the Strategy pattern more appropriate than the Template Method pattern?

• Strategy: When you need to choose between different algorithms at runtime.
• Template Method: When the algorithm structure is fixed, but some steps can be customized by subclasses.

44. Explain the benefits of using the Visitor pattern for adding new functionality.

• It allows you to add new operations to existing object structures without modifying the classes of the objects themselves.

45. What are some anti-patterns to avoid when using design patterns?

• • Answer: Overusing patterns, using the wrong pattern for the problem, creating overly complex solutions.

46. How do design patterns impact code maintainability and extensibility?

• • Answer: Improve code maintainability by making it more modular and easier to understand. Enhance extensibility by providing well-defined interfaces and abstractions.

47. Discuss the importance of choosing the right design pattern for a specific problem.

• • Answer: Using the wrong pattern can lead to inefficient, inflexible, or overly complex solutions.

48. How can you learn and apply design patterns effectively?

• • Answer: Study and understand common design patterns. Analyze existing codebases to identify the patterns used. Practice implementing patterns in your own projects.

49. How do design patterns relate to object-oriented programming principles?

• • Answer: Design patterns are often based on and help to implement object-oriented principles like encapsulation, inheritance, polymorphism, and abstraction.

50. How can you effectively communicate design patterns to other developers?

• • Answer: Use clear and concise language. Draw diagrams and use visual aids. Provide code examples and explain the rationale behind the chosen pattern.