Top Design Patterns Interview Questions

A design pattern is basically a reusable and generalized solution to a common problem that arises during software design and development. Design patterns are not specific to a particular programming language or technology instead, they provide abstract templates or blueprints for solving recurring design-related problems. They help software developers create well-structured, maintainable, and flexible code by promoting best practices and proven solutions.

1. What is a Design Pattern?

A design pattern is a reusable and generalized solution to a common problem that arises during software design and development.

2. What are the types of Design Patterns?

There are four types of Design Patterns.

• Structural patterns
• Creational patterns
• Behavioral patterns
• J2EE patterns

3. What are the advantages of using Design Patterns?

There are various advantages of using design Patterns:

1. The Design Patterns capture software engineering experiences.
2. They are reusable and can be used in multiple projects.
3. They provide transparency to software design.
4. The Design Patterns provide a solution that helps to define the system architecture.

4. What are the types of creational Patterns?

There are five types of creational Patterns :

1. Factory method/Template
2. Abstract Factory
3. Builder
4. Prototype
5. Singleton

5. What are the types of Structural patterns?

The types of creational patterns are as follow :

1. Adapter
2. Bridge
3. Filter
4. Composite
5. Decorator
6. Facade
7. Flyweight
8. Proxy

6. What are the types of Behavioral patterns?

The types of Behavioral Patterns are as follow:

1. Interpreter
2. Template method/ pattern
3. Chain of responsibility
4. Command pattern
5. Iterator pattern
6. Strategy pattern
7. Visitor pattern

7. What is Known as Gang of Four?

The four authors who published the book Design Patterns Elements of Reusable Object-Oriented Software are known as Gang of Four. The name of four authors are Erich Gamma, Ralph Johnson, Richard Hel, and John Vlissides.

8. What is the Singleton pattern, and when would you use it?

The Singleton pattern ensures that a class has only one instance and provides a global point of access to it. It is used when you want to limit object creation for a class to only one instance.

For example :

Java

public class Singleton {
    private static Singleton instance;
    private Singleton() {}  // Private constructor

    public static Singleton getInstance() {
        if (instance == null) {
            instance = new Singleton();
        }
        return instance;
    }
}

9. Explain the Factory Method pattern and provide an example of its use.

The Factory Method pattern defines an interface for creating an object but lets subclasses alter the type of objects that will be created.

Example in python :

Python

from abc import ABC, abstractmethod

class Creator(ABC):
    @abstractmethod
    def factory_method(self):
        pass

    def some_operation(self):
        product = self.factory_method()
        return f"Creator: {product.operation()}"

class ConcreteCreator1(Creator):
    def factory_method(self):
        return ConcreteProduct1()

class ConcreteCreator2(Creator):
    def factory_method(self):
        return ConcreteProduct2()

class Product(ABC):
    @abstractmethod
    def operation(self):
        pass

class ConcreteProduct1(Product):
    def operation(self):
        return "Product 1"

class ConcreteProduct2(Product):
    def operation(self):
        return "Product 2"

10. Describe the Adapter pattern and provide an example of where it can be applied.

The Adapter pattern allows the interface of an existing class to be used as another interface. It’s often used to make existing classes work with others without modifying their source code.

Example:

C#

interface ITarget {
    void Request();
}

class Adaptee {
    public void SpecificRequest() {
        Console.WriteLine("Adaptee's method called");
    }
}

class Adapter : ITarget {
    private Adaptee adaptee = new Adaptee();

    public void Request() {
        adaptee.SpecificRequest();
    }
}

11. Provide a scenario where the Command pattern would be preferable to the Strategy pattern.

The Command pattern is preferable when you want to encapsulate a request as an object with additional metadata, such as the request’s originator or queuing commands for execution. The Strategy pattern, on the other hand, focuses on encapsulating interchangeable algorithms without metadata.

12. Explain the Single Responsibility Principle and its significance in software design.

SRP states that a class should have only one reason to change, meaning it should have only one responsibility or job. This principle encourages modular and maintainable code by reducing the impact of changes in one part of the codebase on other parts.

13. What is the Observer pattern, and how does it enable objects to notify others of changes in state?

The Observer pattern defines a one-to-many relationship between objects, where one object (the subject) maintains a list of its dependents (observers) and notifies them of state changes.

For example:

Java

import java.util.ArrayList;
import java.util.List;

interface Observer {
    void update(String message);
}

class ConcreteObserver implements Observer {
    private String name;

    public ConcreteObserver(String name) {
        this.name = name;
    }

    public void update(String message) {
        System.out.println(name + " received message: " + message);
    }
}

class Subject {
    private List<Observer> observers = new ArrayList<>();

    public void attach(Observer observer) {
        observers.add(observer);
    }

    public void detach(Observer observer) {
        observers.remove(observer);
    }

    public void notifyObservers(String message) {
        for (Observer observer : observers) {
            observer.update(message);
        }
    }
}

14. Describe the Open/Closed Principle and how design patterns support it.

OCP states that software entities (classes, modules, functions) should be open for extension but closed for modification. Design patterns like the Strategy and Decorator patterns allow for adding new functionality without changing existing code, adhering to OCP.

15. How is the Bridge pattern different from the Adapter pattern?

• The Bridge pattern is designed to isolate a class’s interface from its implementation so we can vary or substitute the implementation without changing the client code.
• The motive of the Adapter pattern is to make interfaces of one or more classes to look similar.

16. How does the Dependency Inversion Principle promote loose coupling, and how is it related to design patterns?

DIP encourages high-level modules to depend on abstractions rather than concrete implementations, promoting loose coupling and flexibility. Many design patterns, such as Factory Method and Dependency Injection, implement this principle to enable decoupled and testable code.

17. Provide a real-world example of the Singleton pattern being used in a popular library or framework.

Many libraries and frameworks use the Singleton pattern to manage resources or configuration settings. For example, the Java java.lang.Runtime class is a Singleton that represents the runtime environment.

18. Provide an example where the Strategy pattern is used to switch between different algorithms.

The Strategy pattern is often used in sorting algorithms. For example, a sorting algorithm could be selected at runtime based on user preferences or data characteristics.

19. When should you avoid using design patterns, and what are the potential drawbacks?

Avoid using design patterns when they add unnecessary complexity or when simpler solutions are sufficient. Overusing patterns can lead to code bloat and reduced clarity.

20. How can you ensure that design patterns do not lead to over-engineering or unnecessary complexity?

To prevent over-engineering, apply design patterns judiciously. Choose patterns that directly address the specific problems you’re trying to solve and avoid excessive abstractions or indirection.

21. Provide a scenario where the Decorator pattern is used to enhance the functionality of an existing class in a codebase.

In a text processing application, the Decorator pattern can be used to add formatting, spell-checking, or encryption capabilities to a basic text editor.

22. How can the Command pattern be applied in a user interface (UI) framework?

In a UI framework, the Command pattern is used to encapsulate user actions as objects. These objects can be stored in command history, undone, or redone, enabling features like undo/redo functionality.

23. What is the role of a UML diagram in illustrating design patterns?

UML (Unified Modeling Language) diagrams help visually represent design patterns, classes, relationships, and interactions. They are essential for communicating design ideas and documenting design patterns effectively.

24. How can design patterns help with refactoring existing code?

Design patterns provide well-established solutions to common design problems. When refactoring code, you can identify opportunities to apply these patterns to improve code structure, maintainability, and flexibility.

25. Provide an example where the Strategy pattern is used to switch between different algorithms.

The Strategy pattern is often used in sorting algorithms. For example, a sorting algorithm could be selected at runtime based on user preferences or data characteristics.

Note: These interview questions cover a wide range of topics related to design patterns and software design principles. Preparing answers to these questions will help you demonstrate your understanding of design patterns and their practical applications in interviews