Why Are Design Patterns Essential?

• Code Reusability: They make it easy to reuse solutions, saving time and reducing errors.
• Maintainability: They promote clear, organised code that’s easier to update and maintain.
• Scalability: They allow systems to grow and adapt to changing needs without becoming messy.
• Communication: They give developers a common language to discuss and understand software design.

Categories of Design Patterns

• Creational Patterns: Help with creating objects in flexible ways (e.g., Factory Method, Singleton).
• Structural Patterns: Deal with organising classes and objects to form larger structures (e.g., Adapter, Composite).
• Behavioral Patterns: Manage how objects interact and behave (e.g., Observer, Strategy).

Singleton Pattern

• Usage: Ensure there’s only one instance of a class.
• Example: Logging system with only one logger instance for the entire app.

Factory Method Pattern

• Usage: Delegate object creation to subclasses.
• Example: Pizza restaurant with different types of pizzas made by subclasses.

Abstract Factory Pattern

• Usage: Create families of related objects without specifying their classes.
• Example: GUI toolkit creating buttons for different operating systems.

Builder Pattern

• Usage: Construct complex objects independently from their representation.
• Example: Building a car with various optional features.

Prototype Pattern

• Usage: Create new objects by copying an existing one.
• Example: Drawing app copying complex shapes like houses or trees.

Adapter Pattern

• Purpose: Helps objects with different interfaces work together.
• Example: Imagine connecting a new phone charger (with a different plug) to an old outlet using an adapter.

Bridge Pattern

• Purpose: Separates abstraction from implementation to allow flexibility.
• Example: Think of a remote-controlled car where you can change the type of remote (abstraction) without changing the car’s internals (implementation).

Composite Pattern

• Purpose: Lets you treat individual objects and groups of objects the same way.
• Example: In a game, you can treat individual characters and entire groups of characters as “entities” without distinguishing between them.

Decorator Pattern

• Purpose: Adds new features to objects without changing their structure.
• Example: Decorating a plain cake with different frosting and toppings to create different flavors without altering the cake itself.

Facade Pattern

• Purpose: Provides a simple interface to a complex system.
• Example: Using a smartphone’s touchscreen to control various internal components without needing to understand how each component works.

Flyweight Pattern

• Purpose: Minimises memory usage by sharing common parts between objects.
• Example: Sharing the same background image across multiple windows in a graphic design software to save memory.

Proxy Pattern

• Purpose: Acts as a substitute for another object to control access.
• Example: Using a security guard to control access to a building, where visitors interact with the guard instead of directly with the building’s residents.

Observer Pattern

• When to Use: When one object needs to notify others about changes in its state.
• Benefits: Helps maintain consistency between related objects without tightly coupling them.

Strategy Pattern

• When to Use: When you want to switch between different algorithms dynamically.
• Benefits: Allows for easy algorithm interchangeability and promotes code reuse.

Command Pattern

• When to Use: When you need to encapsulate requests or operations as objects.
• Benefits: Enables queuing of requests, undoable actions, and logging.

Template Method Pattern

• When to Use: When you want to define the outline of an algorithm but allow subclasses to provide specific implementations for certain steps.
• Benefits: Promotes code reuse and simplifies algorithm maintenance.

State Pattern

• When to Use: When an object’s behavior depends on its state and must change dynamically.
• Benefits: Encapsulates state-specific behavior and simplifies state transitions.

Iterator Pattern

• When to Use: When you want to traverse different types of collections uniformly.
• Benefits: Provides a consistent way to access elements without exposing internal structure.

Mediator Pattern

• When to Use: When you have a complex set of interactions between objects.
• Benefits: Promotes loose coupling and simplifies communication between objects.

Model-View-Controller (MVC) Pattern

• Example: Websites like Facebook or Instagram use MVC to separate data (model), presentation (view), and logic (controller) for easier management.
• Impact: MVC makes it easier to change or add features without breaking everything (maintainability), allows the site to handle more users without slowing down (scalability), and makes it easier to add new features in the future (extensibility).

Singleton Pattern

• Example: Apps often use Singleton to ensure there’s only one instance of a resource, like a database connection.
• Impact: Singleton helps keep things organised (maintainability), ensures resources are used efficiently (scalability), but can make code hard to change if overused (extensibility).

Strategy Pattern

• Example: Sorting functions in apps can use the Strategy pattern to switch between different sorting methods.
• Impact: Strategy keeps code flexible (maintainability), allows new methods to be added easily (scalability), and makes it easier to understand and change code later (extensibility).

Observer Pattern

• Example: Chat apps use Observer to notify users when new messages arrive.
• Impact: Observer keeps parts of the app separate (maintainability), allows for easy updates without breaking other parts (scalability), and makes it easy to add new features (extensibility).

Best Practices

• Know Your Problem: Understand the problem before applying a design pattern. Make sure the pattern fits the problem you’re trying to solve.
• Keep It Simple: Don’t complicate things with unnecessary design patterns. Sometimes a straightforward solution works best.
• Follow Standards: Stick to common naming and structure conventions. Consistency makes your code easier to understand.
• Document Your Choices: Explain why you’re using a particular design pattern and how it solves the problem. This helps others understand your code.
• Adapt as Needed: Refactor your code when requirements change. Design patterns should evolve with your software.

Pitfalls to Avoid

1. Overthinking: Don’t use design patterns excessively. Keep your codebase simple and easy to understand.
2. Optimising Too Soon: Focus on readability and maintainability first. Only optimise for performance when needed.
3. Ignoring Your Domain: Make sure design patterns fit your specific requirements. Don’t force them if they don’t align with your domain.
4. Ignoring Team Knowledge: Ensure everyone on the team understands the design patterns being used. Consistency is key.
5. Ignoring Evolution: Be willing to change your design as requirements evolve. Don’t stick to a pattern if it no longer fits.