What are the different types of design patterns?

Design patterns in software engineering are generally categorized into three primary types: Creational, Structural, and Behavioral. Each type addresses a specific aspect of software design, from object creation and structure to behavior and communication. Here’s an in-depth look at each category:

1. Creational Patterns

Creational patterns are concerned with the process of object creation. They provide solutions to instantiate objects in a manner that enhances flexibility, scalability, and reusability. Instead of directly instantiating objects, creational patterns allow for the creation of objects based on specific criteria, streamlining the object creation process.

Key Creational Patterns:

• Singleton: Ensures that a class has only one instance and provides a global point of access to that instance.
• Factory Method: Defines an interface for creating an object, but lets subclasses alter the type of objects that will be created.
• Abstract Factory: Provides an interface to create families of related or dependent objects without specifying their concrete classes.
• Builder: Separates the construction of a complex object from its representation, enabling the same construction process to create different representations.
• Prototype: Creates new objects by copying an existing object, which serves as a prototype, rather than creating objects from scratch.

When to Use:

• When you need flexible and reusable object creation mechanisms.
• When the system should be independent of the way its objects are created or represented.

2. Structural Patterns

Structural patterns focus on organizing different classes and objects to form larger, cohesive structures. They help compose interfaces and object implementations to enable efficient use of resources while maintaining code flexibility.

Key Structural Patterns:

• Adapter: Allows incompatible interfaces to work together by converting one interface to another that a client expects.
• Composite: Composes objects into tree structures to represent part-whole hierarchies, letting clients treat individual objects and compositions uniformly.
• Decorator: Dynamically adds new responsibilities to an object without altering its structure, providing a flexible alternative to subclassing.
• Facade: Provides a simplified interface to a complex system of classes, hiding the complexity of the underlying components.
• Proxy: Provides a surrogate or placeholder for another object, controlling access to it (useful for lazy initialization, logging, or access control).
• Bridge: Separates an object’s abstraction from its implementation, allowing the two to vary independently.
• Flyweight: Reduces the cost of creating and manipulating a large number of similar objects by sharing common parts among them.

When to Use:

• When building complex systems that need to be simplified or structured for easy management.
• When extending functionality to classes without extensive subclassing or modifying existing code.

3. Behavioral Patterns

Behavioral patterns are concerned with how objects communicate and interact. They help define the ways that objects exchange information and responsibilities, focusing on how to improve collaboration and ensure flexibility.

Key Behavioral Patterns:

• Observer: Defines a one-to-many relationship where multiple observers are notified of changes to a subject. Common in event handling systems.
• Strategy: Encapsulates a family of algorithms and makes them interchangeable. The algorithm can vary independently from the clients that use it.
• Command: Encapsulates a request as an object, thereby allowing for parameterization of clients with different requests, queuing of requests, and logging.
• Iterator: Provides a way to access elements of an aggregate object sequentially without exposing its underlying structure.
• State: Allows an object to alter its behavior when its internal state changes, making it appear as if the object’s class has changed.
• Mediator: Defines an object that centralizes and controls how objects interact, promoting loose coupling.
• Chain of Responsibility: Passes a request along a chain of handlers, allowing multiple objects the chance to process the request.
• Template Method: Defines the skeleton of an algorithm, letting subclasses override specific steps without changing the algorithm’s structure.
• Visitor: Allows you to add further operations to objects without modifying them by separating the algorithm from the object structure.

When to Use:

• When defining complex communication or interaction protocols between objects.
• When decoupling the way algorithms are applied to data structures from the data structures themselves.

Quick Summary Table

Examples of Applying Design Patterns

1. Singleton Pattern in Logging:

   • Ensures only one instance of a logging class exists, allowing centralized control over log management in the system.

2. Adapter Pattern in File Conversion:

   • Adapts a file-reading class that reads .txt files to work with a .csv file parser without changing the original class.

3. Observer Pattern in Event Handling:

   • In a UI framework, when a button is clicked, all subscribed listeners (observers) are notified and react accordingly.

4. Strategy Pattern in Payment Processing:

   • Enables interchangeable payment methods (credit card, PayPal, crypto) by defining a strategy interface for different payment algorithms.

5. Facade Pattern in Complex Subsystems:

   • Simplifies complex interactions with a subsystem, such as an external API, by providing a simplified interface to clients.

Recommended Resources

• Books:

  • "Design Patterns: Elements of Reusable Object-Oriented Software" by the Gang of Four (GoF) is the classic reference on design patterns.
  • "Head First Design Patterns" by Eric Freeman and Elisabeth Robson provides a beginner-friendly introduction to design patterns with practical examples.

• Online Courses:

  • DesignGurus.io offers courses tailored for mastering design patterns:
    • Grokking the Object-Oriented Design Interview: Learn to approach and solve design challenges, focusing on object-oriented design principles and common design patterns.
    • Grokking the System Design Interview: Explore system design principles, including the effective use of design patterns in building scalable systems.

• Practice Platforms:

  • Refactoring.Guru: Offers a detailed guide to each design pattern with real-world examples and code snippets in multiple languages.
  • GitHub: Explore repositories that provide sample implementations of design patterns in various programming languages.

Conclusion

Understanding and applying design patterns can significantly improve the flexibility, scalability, and maintainability of your software. Each design pattern provides a solution to a recurring problem in software design, helping developers build complex systems more efficiently. By mastering creational, structural, and behavioral patterns, software engineers can create more organized, efficient, and reusable codebases, making these patterns essential tools in the field of software development.