A comprehensive guide to proven, reusable solutions for common software design challenges.
Software development often feels like solving a complex puzzle. While Object-Oriented Programming (OOP) provides a powerful foundation for building scalable systems, developers regularly face recurring design challenges that can slow progress and introduce technical debt.
Design patterns are time-tested, reusable blueprints that address these challenges elegantly. They are not rigid algorithms — they are flexible, language-agnostic strategies that help you build code that is:
- 🔍 Readable — Communicate intent clearly to your team
- 🔧 Maintainable — Easier to update and debug over time
- 📦 Modular — Loosely coupled, highly cohesive components
- 🚀 Extensible — Adapt to new requirements with minimal friction
Design patterns are organized into three foundational categories, as defined by the Gang of Four (GoF):
| Category | Focus | Count |
|---|---|---|
| 🔁 Creational | Object creation mechanisms | 4 patterns |
| 🔄 Structural | Object composition & relationships | 7 patterns |
| 🔁 Behavioral | Object communication & responsibility | 11 patterns |
Concerned with the way objects are created, aiming to increase flexibility and reuse of existing code.
| Pattern | Intent |
|---|---|
| 🔹 Singleton | Ensure a class has only one instance and provide a global point of access to it. |
| 🔹 Factory Method | Define an interface for creating an object, but let subclasses decide which class to instantiate. |
| 🔹 Builder | Separate the construction of a complex object from its representation, enabling the same process to produce different results. |
| 🔹 Prototype | Create new objects by cloning an existing instance — ideal when object creation is costly. |
Concerned with how classes and objects are composed to form larger, more flexible structures.
| Pattern | Intent |
|---|---|
| 🔸 Adapter | Convert the interface of a class into another interface that clients expect — bridging incompatibilities. |
| 🔸 Facade | Provide a unified, simplified interface to a complex subsystem. |
| 🔸 Proxy | Provide a surrogate or placeholder to control access to another object. |
| 🔸 Decorator | Attach additional responsibilities to an object dynamically, without altering its core structure. |
| 🔸 Composite | Compose objects into tree structures to represent part-whole hierarchies uniformly. |
| 🔸 Bridge | Decouple an abstraction from its implementation, allowing each to evolve independently. |
| 🔸 Flyweight | Minimize memory usage by sharing as much data as possible with similar objects. |
Concerned with communication, responsibility, and algorithms between objects.
| Pattern | Intent |
|---|---|
| 🔹 Strategy | Define a family of algorithms, encapsulate each one, and make them interchangeable at runtime. |
| 🔹 Template Method | Define the skeleton of an algorithm in a base class, deferring some steps to subclasses. |
| 🔹 Iterator | Provide a way to access elements of a collection sequentially without exposing its underlying structure. |
| 🔹 Observer | Establish a one-to-many dependency so that when one object changes state, all dependents are notified automatically. |
| 🔹 State | Allow an object to alter its behavior when its internal state changes — the object will appear to change its class. |
| 🔹 Command | Encapsulate a request as an object, enabling parameterization, queuing, logging, and undoable operations. |
| 🔹 Chain of Responsibility | Pass requests along a chain of handlers, where each decides to process or forward the request. |
| 🔹 Interpreter | Given a language, define a representation for its grammar and an interpreter to handle sentences. |
| 🔹 Mediator | Define an object that encapsulates how a set of objects interact, promoting loose coupling. |
| 🔹 Memento | Capture and externalize an object's internal state so it can be restored later, without violating encapsulation. |
| 🔹 Visitor | Represent an operation to be performed on elements of an object structure, without changing their classes. |
Without Patterns With Patterns
───────────────────────────────── ─────────────────────────────────
❌ Reinventing solutions each time ✅ Reuse proven, battle-tested designs
❌ Tight coupling between classes ✅ Loosely coupled, modular architecture
❌ Hard to communicate design ideas ✅ Shared vocabulary for your team
❌ Rigid code that resists change ✅ Extensible systems that adapt easily
❌ Growing technical debt ✅ Cleaner, more maintainable codebases
Not sure which pattern to use? Here's a quick decision guide:
- Need only one instance of a class? →
Singleton - Creating objects without knowing the exact type? →
Factory Method - Dealing with incompatible interfaces? →
Adapter - Want to add features without subclassing? →
Decorator - Need to notify multiple objects of a change? →
Observer - Want to swap algorithms at runtime? →
Strategy - Need to undo/redo operations? →
Command+Memento - Simplifying a complex API? →
Facade
Contributions, issues, and feature requests are welcome!
- Fork this repository
- Create a new branch (
git checkout -b feature/add-pattern) - Commit your changes (
git commit -m 'Add example for Observer pattern') - Push to the branch (
git push origin feature/add-pattern) - Open a Pull Request
Please ensure your examples include clear comments and follow consistent coding style.
I'm always happy to connect, collaborate, or discuss software design!
| Platform | Link |
|---|---|
| 🌐 Website | www.sajithjeewantha.com |
| linkedin.com/in/sajith-jeewantha |
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"Design patterns are not a silver bullet — they are a shared language for building great software."