🧱 Design Patterns

Software design patterns are reusable solutions to common design problems in software development. They provide proven ways to structure classes, objects, and interactions so your code becomes more maintainable, flexible, and robust. Patterns don’t give you finished code—they give you templates for solving recurring problems.

Software design patterns — definition

Software design patterns are generalized, reusable solutions to common problems in object‑oriented and component‑based software design. They describe how to structure classes and objects, how they should interact, and how responsibilities should be distributed.

They answer:

How should objects collaborate?
How do we avoid duplicated logic?
How do we make code flexible and maintainable?

Why design patterns matter

Patterns help you:

Improve maintainability
Reduce complexity
Increase flexibility and extensibility
Standardize solutions across teams
Communicate design ideas quickly
Avoid reinventing the wheel

Patterns are especially important in large systems where consistency matters.

Categories of design patterns

1. Creational patterns

Deal with object creation and lifecycle.

Singleton — one instance globally
Factory Method — subclasses decide which object to create
Abstract Factory — create families of related objects
Builder — step‑by‑step object construction
Prototype — clone existing objects

2. Structural patterns

Define how classes and objects are composed.

Adapter — convert one interface into another
Decorator — add behavior dynamically
Facade — simplify complex subsystems
Composite — treat groups of objects like individuals
Proxy — control access to another object
Bridge — separate abstraction from implementation
Flyweight — share small objects to save memory

3. Behavioral patterns

Define how objects communicate and assign responsibilities.

Strategy — interchangeable algorithms
Observer — event subscription model
Command — encapsulate actions as objects
Mediator — centralize communication
State — change behavior based on internal state
Chain of Responsibility — pass requests along a chain
Template Method — define algorithm skeleton
Iterator — traverse collections uniformly
Memento — capture and restore object state
Interpreter — define grammar and evaluation
Visitor — separates algorithms from the object structure they operate on

How design patterns differ from architecture

Patterns operate at the code level, not the system level.

Architecture → high‑level structure (services, modules, boundaries)
Design patterns → low‑level structure (classes, objects, interactions)

Patterns help implement architecture cleanly.

When to use design patterns

Use patterns when:

A problem repeats across modules
You need flexibility or extensibility
You want to enforce clean separation of concerns
You want to avoid duplicated logic
You want to communicate design clearly to other engineers

Avoid patterns when they add unnecessary complexity.

Example

If you apply design patterns to a Daily Scrum application:

Strategy → different summary generation algorithms
Observer → notify subscribers when tasks change
Command → encapsulate “Generate PDF” or “Send Notification” actions
Factory → create different PDF exporters
Decorator → add logging or caching to services
Facade → simplify access to the summary + PDF + notification subsystems

Patterns help keep your code modular and maintainable.

Takeaway

Software design patterns are reusable solutions to common design problems. They help structure classes and interactions in a way that improves maintainability, flexibility, and clarity. They are essential tools for clean, scalable software design.