Design patterns Interview Questions
What are design patterns, and why are they important in software development?

Answer: Design patterns are reusable solutions to common software design problems. They promote best practices, code maintainability, and help developers communicate effectively. Patterns save time and ensure robust, well-structured code.

Can you name and explain the three main categories of design patterns?

Answer: The three main categories are Creational (dealing with object creation mechanisms), Structural (dealing with object composition), and Behavioral (dealing with object collaboration and responsibility).

Explain the Singleton design pattern and provide a use case where it's applicable.

Answer: Singleton ensures a class has only one instance and provides a global point of access. It's used for logging, database connections, and caching where a single instance is needed for resource management.

What is the Factory Method design pattern, and how does it differ from the Abstract Factory pattern?

Answer: The Factory Method creates objects without specifying the exact class to instantiate. The Abstract Factory provides an interface for creating families of related or dependent objects. Factory Method deals with one product, while Abstract Factory deals with multiple products.

Explain the Observer design pattern and provide an example of its use in real-world applications.

Answer: The Observer pattern defines a one-to-many dependency between objects. It's used in scenarios like event handling, where multiple objects (observers) need to be notified when a state changes in a subject (observable) object.

What is the purpose of the Adapter design pattern, and when would you use it?

Answer: The Adapter pattern allows the interface of an existing class to be used as another interface. It's used when you want to make an existing class compatible with a new interface without modifying its source code.

Can you explain the Decorator pattern and provide an example of how it can be applied in software development?

Answer: The Decorator pattern attaches additional responsibilities to objects dynamically. It's useful for adding features or behaviors to classes without subclassing. An example is adding functionality like encryption or compression to file I/O operations.

What is the Strategy design pattern, and how does it help in achieving better code maintainability and flexibility?

Answer: The Strategy pattern defines a family of algorithms, encapsulates them, and makes them interchangeable. It allows you to change algorithms at runtime, making code more maintainable and flexible.

Explain the Composite design pattern and provide an example of its use in graphical user interfaces.

Answer: The Composite pattern allows clients to treat individual objects and compositions of objects uniformly. In GUIs, it's used to represent hierarchical structures, such as nested menus or tree views.

What is the MVC (Model-View-Controller) architectural pattern, and how does it promote separation of concerns in application design?

Answer: MVC separates an application into Model (data and logic), View (user interface), and Controller (user input handling). It promotes modularity and separation of concerns, making applications easier to maintain and scale.

Can you name a design pattern that focuses on reducing the number of connections between objects in a system?

Answer: The Flyweight pattern aims to minimize memory usage or computational expenses by sharing as much as possible among related objects, reducing the number of individual objects.

What design pattern is commonly used for undo/redo functionality in applications?

Answer: The Memento pattern is often used for implementing undo/redo functionality by capturing and restoring an object's internal state.

Explain the Chain of Responsibility pattern and its role in handling requests or tasks in a hierarchical manner.

Answer: The Chain of Responsibility pattern passes a request along a chain of handlers. Each handler decides whether to process the request or pass it to the next handler in the chain. It's used for tasks like request filtering or event propagation.