Common Software Architecture Design Patterns Interview Questions

What is a design pattern, and why are they important in software architecture?

• Design patterns are standardized solutions to common problems in software design. They provide a template for solving issues related to object creation, interaction, and responsibility distribution. Understanding these patterns helps architects and developers create more flexible, reusable, and maintainable code.

Can you explain the Singleton pattern and give an example of where it might be used?

• The Singleton pattern ensures that a class has only one instance and provides a global point of access to that instance. It is often used in scenarios where a single instance of a class is needed to coordinate actions across the system, such as in configuration management or logging systems.

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

• The Factory Method pattern defines an interface for creating an object but allows subclasses to alter the type of objects that will be created. The Abstract Factory pattern, on the other hand, provides an interface for creating families of related or dependent objects without specifying their concrete classes. Essentially, the Factory Method is about creating a single product, while the Abstract Factory is about creating a suite of products.

Describe the Observer pattern and provide a use case where it would be appropriate.

• The Observer pattern defines a one-to-many dependency between objects, where a change in one object results in updates to its dependent objects. This pattern is useful in scenarios where a change in one part of an application needs to be reflected across other parts, such as in user interface frameworks where multiple views need to be updated when the underlying model changes.

What is the Strategy pattern, and how can it improve the flexibility of an application?

• The Strategy pattern defines a family of algorithms, encapsulates each one, and makes them interchangeable. The pattern allows the algorithm to vary independently from clients that use it. This enhances flexibility as it enables the selection of an algorithm at runtime, rather than hardcoding it into the application.

Explain the Decorator pattern and how it differs from subclassing.

• The Decorator pattern allows behavior to be added to individual objects, either statically or dynamically, without affecting the behavior of other objects from the same class. This pattern provides a flexible alternative to subclassing for extending functionality. Unlike subclassing, which involves creating a new class for each variation, decorators allow you to combine various functionalities dynamically.

What is the Adapter pattern, and how does it facilitate compatibility between incompatible interfaces?

• The Adapter pattern acts as a bridge between two incompatible interfaces. It allows classes to work together that couldn't otherwise due to incompatible interfaces. By providing a wrapper around the existing interface, the Adapter pattern enables the integration of different systems or components without modifying their existing code.

Can you describe the Template Method pattern and provide an example of how it might be used in software design?

• The Template Method pattern defines the skeleton of an algorithm in a base class but lets subclasses redefine certain steps of the algorithm without changing its structure. This pattern is useful when you have a sequence of steps that need to be followed, but some steps can vary. For example, in a data processing application, you might have a general template for processing data but allow specific implementations to handle different data sources.

How does the Proxy pattern work, and in what scenarios might it be applied?

• The Proxy pattern provides a surrogate or placeholder for another object to control access to it. Proxies can be used to control access, implement lazy initialization, or provide additional functionality such as logging. For example, a virtual proxy might be used to delay the loading of a large object until it is actually needed.

What is the Command pattern, and how does it decouple the sender and receiver of a request?

• The Command pattern encapsulates a request as an object, thereby allowing for parameterization of clients with queues, requests, and operations. It also supports undoable operations. This pattern decouples the sender of a request from the receiver, enabling more flexible command handling and easier extensibility.