What are Design Patterns? Learn about 23 software design patterns to code more effectively.

Definition

A Design Pattern is a proven and optimized solution used to address recurring problems in software design. It is not a programming language but rather a technique in Object-Oriented Programming (OOP) that can be flexibly applied in most languages such as Java, C#, JavaScript, and many others.
Design Patterns help developers solve problems systematically, save time, and avoid common errors. Instead of coming up with solutions from scratch, you can apply "design patterns" that have been validated by the community to improve efficiency, maintainability, and scalability.
However, to learn and apply Design Patterns effectively, a strong foundation in programming is essential—particularly a solid understanding of OOP concepts. If you're a beginner and unfamiliar with terms like class, object, inheritance, and polymorphism, it’s recommended that you focus on learning OOP first. Once you're comfortable with OOP, working with Design Patterns becomes much easier and more beneficial for your development skills.

How Design Patterns Support Developers

Accelerated Software Development

During application development, Design Patterns serve as “mental toolkits” that help developers efficiently solve common software design issues. Even if you haven’t encountered a problem yet, understanding Design Patterns is still crucial. They train your design thinking and teach you to apply OOP principles in a structured, flexible, and systematic way.
In the long run, Design Patterns help you write more optimal, maintainable code and improve your ability to read, communicate, and collaborate with other developers in large projects.

Clear Code & Easier Teamwork

In addition to providing solutions, Design Patterns act as a shared language among developers. When one developer mentions a pattern, others can immediately understand the design intent, overall structure, and implementation approach. This reduces time spent on analysis and discussion, optimizing the development process.
Using design patterns is like using technical keywords—concise yet packed with meaning—helping teams work efficiently, consistently, and with fewer misunderstandings during system design.

Code Reusability

From a project development perspective, Design Patterns bring clear benefits in terms of code reuse and extensibility. They offer practical, field-tested solutions to common design problems.
Instead of reinventing the wheel with potentially suboptimal or hard-to-maintain solutions, developers can use design patterns as guiding principles—well-documented and widely accepted by the programming community. This not only saves development time but also ensures long-term stability, maintainability, and scalability of the software.

Fewer Bugs & Easier Upgrades

Besides improving efficiency, using Design Patterns reduces risks during software development. Since they rely on proven solutions, developers can avoid subtle design flaws that might become serious issues later.
Moreover, code structured around design patterns typically has a clear architecture and separation of concerns, making it easier to upgrade, extend, and maintain—especially important for long-term or large-team projects.

Categories of Design Patterns

There are many Design Patterns in use today, but the most widely recognized and systematically organized are the 23 classic patterns introduced in the famous book "Design Patterns: Elements of Reusable Object-Oriented Software" by the Gang of Four (GoF).
These patterns are grouped into three main categories based on their purpose:

1. Creational Patterns

These handle object creation in a flexible and optimized way, separating creation logic from usage.

Includes 5 patterns:

• Singleton: Ensures a class has only one instance and provides a global access point to it.
• Factory Method: Defines an interface for creating an object, allowing subclasses to decide which class to instantiate.
• Abstract Factory: Provides an interface for creating families of related objects without specifying their concrete classes.
• Builder: Separates the construction of a complex object from its representation so the same construction process can create different representations.
• Prototype: Creates new objects by copying an existing instance (cloning).

2. Structural Patterns

Focus on how to combine classes and objects to form larger structures that are flexible and extensible.

Includes 7 patterns:

• Adapter: Converts the interface of a class into another interface that clients expect.
• Bridge: Separates an abstraction from its implementation so the two can vary independently.
• Composite: Composes objects into tree structures to represent part-whole hierarchies and treat individual and composite objects uniformly.
• Decorator: Adds new functionality to an object dynamically without altering its structure.
• Facade: Provides a simplified interface to a complex subsystem.
• Flyweight: Reduces memory usage by sharing as much data as possible with similar objects.
• Proxy: Provides a surrogate or placeholder for another object to control access to it.

3. Behavioral Patterns

Describe how objects interact and communicate with one another.

Includes 11 patterns:

• Interpreter: Builds an interpreter to parse and execute a specific language.
• Template Method: Defines the skeleton of an algorithm, deferring some steps to subclasses.
• Chain of Responsibility: Passes a request along a chain of handlers until one of them handles it.
• Command: Encapsulates a request as an object, allowing for parameterization, queuing, and undo functionality.
• Iterator: Provides a way to access elements of a collection sequentially without exposing its internal representation.
• Mediator: Defines an object that centralizes communication between other objects.
• Memento: Captures and restores an object’s internal state without violating encapsulation.
• Observer: Allows an object to notify other objects when its state changes.
• State: Allows an object to alter its behavior when its internal state changes.
• Strategy: Defines a family of algorithms and makes them interchangeable.
• Visitor: Allows you to define new operations on a set of objects without changing their classes.

References

• Refactoring.Guru – Design Patterns
• Design Patterns for Dummies – Steve Holzner, PhD
• Head First Design Patterns – Eric Freeman
• Design Patterns: Elements of Reusable Object-Oriented Software – Gang of Four
• Dive into Design Patterns