Design patterns

A design pattern is a reusable solution to a common programming problem. It helps make your code more structured, maintainable, and scalable.

• Avoid rewriting code for the same problems.
• Improve code readability and reusability.
• Make your code more flexible to changes.

Types of design patterns: There are three main types of design patterns:

• Creational Patterns: How objects are created.
• Structural Patterns: How objects are structured & connected.
• Behavioral Patterns: How objects interact & communicate.

Creational design patterns (control object creation)

These patterns provide better ways to control object creation, in a flexible way.

Pattern	What it Does	Example Use Case
Singleton	Ensures only one instance of a class exists.	Database connections, Logging
Factory Method	A method creates objects instead of new keyword.	Creating different payment methods (CreditCard, PayPal)
Abstract Factory	A factory that creates factories of related objects.	UI toolkit for Windows & Mac
Builder	Builds complex objects step by step.	Creating a Burger with different toppings
Prototype	Copies an existing object instead of creating a new one.	Copying a user profile

Structural Design Patterns (control class relationships)

These patterns deal with how objects are connected and structured.

Pattern	What it Does	Example Use Case
Adapter	Converts one interface into another.	Using an old API in a new system
Bridge	Separates abstraction from implementation.	UI themes with different rendering engines
Composite	Treats a group of objects like a single object.	Folder structure in Windows Explorer
Decorator	Adds extra behavior to objects dynamically.	Adding extra features to a car (e.g., Sunroof)
Facade	Provides a simple interface to a complex system.	A single API call to simplify multiple services
Flyweight	Uses shared objects to save memory.	Text editors where characters share styles
Proxy	Controls access to an object.	Security check before accessing an object

Behavioral Design Patterns (control object communication)

These patterns define how objects interact with each other.

Pattern	What it Does	Example Use Case
Chain of Responsibility	Passes a request through a chain of handlers.	Support ticket escalation (Level 1 → Level 2 → Manager)
Command	Encapsulates a request as an object.	Undo/Redo in text editors
Interpreter	Defines a grammar for interpreting language.	Math expression evaluator
Iterator	Provides a way to loop through a collection without exposing details.	Custom collections in C#
Mediator	Controls communication between multiple objects.	Chatroom where users communicate via a server
Memento	Saves an object's state for undo functionality.	Text editor undo feature
Observer	Notifies multiple objects when one object changes.	Stock price alerts
State	Changes an object's behavior based on state.	Traffic light system (Red, Yellow, Green)
Strategy	Allows switching between different algorithms.	Sorting (BubbleSort, QuickSort)
Template	Method Defines the skeleton of an algorithm in a base class.	Cooking recipes
Visitor	Adds extra functionality without modifying existing classes.	Applying a discount to different products