What Are Design Patterns?

Design patterns are reusable, battle-tested solutions to common problems in software design.

Think of them like blueprints — not finished code you copy-paste, but a template you adapt to your situation.

Real-World Analogy

A chef doesn’t invent a new recipe every time someone orders an omelette. They follow a proven pattern — crack eggs, whisk, cook — and tweak it for the customer’s taste.

Design patterns work the same way: proven solutions you customize for your app.

The Gang of Four (GoF)

In 1994, four authors (Erich Gamma, Richard Helm, Ralph Johnson, John Vlissides) published Design Patterns: Elements of Reusable Object-Oriented Software. They cataloged 23 patterns divided into 3 categories.

The 3 Categories

flowchart TB
    GoF[Gang of Four<br/>23 Patterns] --> Creational[🏗️ Creational<br/>How objects are created]
    GoF --> Structural[🔧 Structural<br/>How objects are composed]
    GoF --> Behavioral[🔄 Behavioral<br/>How objects communicate]

    Creational --> S1[Singleton]
    Creational --> F1[Factory Method]
    Creational --> AF[Abstract Factory]
    Creational --> B1[Builder]
    Creational --> P1[Prototype]

    Structural --> Ad[Adapter]
    Structural --> Dec[Decorator]
    Structural --> Fac[Facade]
    Structural --> Pr[Proxy]
    Structural --> Com[Composite]
    Structural --> Br[Bridge]
    Structural --> Fly[Flyweight]

    Behavioral --> Ob[Observer]
    Behavioral --> St[Strategy]
    Behavioral --> Cmd[Command]
    Behavioral --> It[Iterator]
    Behavioral --> Sta[State]
    Behavioral --> TM[Template Method]
    Behavioral --> CoR[Chain of Resp.]
    Behavioral --> Med[Mediator]
    Behavioral --> Mem[Memento]
    Behavioral --> Vis[Visitor]

    style GoF fill:#7c3aed,color:#fff
    style Creational fill:#06b6d4,color:#fff
    style Structural fill:#f59e0b,color:#fff
    style Behavioral fill:#ef4444,color:#fff

🏗️ Creational Patterns (5)

How objects are created.

Instead of new everywhere, creational patterns give you flexible object creation.

Pattern	What It Does
Singleton	One shared instance for the whole app
Factory Method	Let subclasses decide which class to instantiate
Abstract Factory	Families of related objects without specifying concrete classes
Builder	Construct complex objects step by step
Prototype	Clone existing objects instead of creating new ones

🔧 Structural Patterns (7)

How objects are composed.

Structural patterns assemble objects into larger structures while keeping them flexible.

Pattern	What It Does
Adapter	Make incompatible interfaces work together
Decorator	Add behavior to an object without changing its class
Facade	Provide a simple interface to a complex subsystem
Proxy	Control access to another object
Composite	Treat individual and group objects the same way
Bridge	Separate abstraction from implementation
Flyweight	Share data across many objects to save memory

🔄 Behavioral Patterns (11)

How objects communicate.

Behavioral patterns manage algorithms, responsibilities, and communication between objects.

Pattern	What It Does
Observer	Notify multiple objects when something changes
Strategy	Swap algorithms at runtime
Command	Wrap a request as an object
Iterator	Traverse a collection without exposing internals
State	Change behavior when internal state changes
Template Method	Define skeleton, let subclasses fill details
Chain of Responsibility	Pass a request along a chain of handlers
Mediator	Centralize complex communication
Memento	Save and restore object state
Visitor	Add new operations without changing classes

Why Use Design Patterns?

Reason	Explanation
🧠 Proven	Tested by thousands of developers over decades
🗣️ Shared vocabulary	”Let’s use an Observer here” — everyone understands
🔧 Maintainable	Loosely coupled, easy to change code
🚫 Avoid reinvention	Don’t solve problems that have already been solved

When NOT to Use

Over-engineering: Don’t add a Factory for a single class
Simple problems: A simple function is better than a pattern
Performance-critical: Some patterns add overhead (Proxy, Decorator)

In Simple Words

Design patterns are proven blueprints for common coding problems
There are 23 classic patterns from the Gang of Four
They’re grouped into 3 categories: Creational, Structural, Behavioral
They give you a shared vocabulary with other developers
Use them when the problem fits — but don’t force them