Software Engineering basics - Design Patterns

Design Patterns

Definition:
Design patterns are proven, reusable solutions to common software design problems. They provide a template or blueprint that developers can follow to solve a recurring issue in software design, without having to reinvent the solution each time.

Think of it like architectural patterns in building design—you don’t design a staircase from scratch every time; you use a known design that works.

Why Design Patterns Matter

1. Reusability – Use tried-and-tested solutions instead of reinventing the wheel.

2. Maintainability – Makes code easier to read, understand, and modify.

3. Scalability & Flexibility – Helps software adapt to changing requirements.

4. Communication – Provides a common vocabulary for developers (e.g., “We’re using a Singleton here”).

Categories of Design Patterns

1. Creational Patterns – Deal with object creation mechanisms.
   Ensure objects are created in a way that is flexible and reusable.

   • Singleton: Ensures only one instance of a class exists.

   • Factory Method: Creates objects without specifying the exact class.

   • Builder: Constructs complex objects step by step.

   • Prototype: Clones existing objects instead of creating new ones.

   • Abstract Factory: Produces families of related objects.

2. Structural Patterns – Deal with how classes and objects are composed.
   Help form large structures from smaller components.

   • Adapter: Allows incompatible interfaces to work together.

   • Decorator: Adds responsibilities to objects dynamically.

   • Facade: Provides a simplified interface to a complex system.

   • Proxy: Controls access to an object.

   • Composite: Treats individual objects and compositions uniformly.

3. Behavioral Patterns – Deal with object communication and responsibility.
   Define how objects interact and distribute responsibilities.

   • Observer: Notifies dependent objects when a state changes.

   • Strategy: Encapsulates algorithms for interchangeable use.

   • Command: Encapsulates a request as an object.

   • Iterator: Provides a way to access elements sequentially.

   • Mediator: Defines an object that controls communication between others.

Key Points

• Design patterns are not code you copy-paste; they are concepts and templates.

• They improve code readability, maintainability, and flexibility.

• Patterns are often combined to solve more complex design problems.