Design patterns are proven solutions to common software design problems. In this guide, we’ll explore the most widely used patterns in real-world Java applications, organized into three core categories:
- Creational Patterns – for object creation
- Structural Patterns – for organizing code
- Behavioral Patterns – for managing interactions and responsibilities
🏗️ Creational Patterns
✅ Singleton Pattern
Where to use: Use Singleton when you want to ensure only one instance of a class exists in your application — like a shared configuration object, logger, or cache manager.
Why: Singleton provides controlled access to a single, globally shared object. It prevents multiple instantiations and ensures consistent state throughout the app, especially in stateless web apps or services.
✅ Factory Method Pattern
Where to use: Use Factory Method when your code needs to instantiate objects, but you want to defer the actual class to be instantiated to subclasses or external logic.
Why: It decouples object creation from the client logic, making your code more modular and flexible. It’s widely used in frameworks (like Spring) where beans are created dynamically based on configuration.
✅ Builder Pattern
Where to use: Use Builder when creating complex objects with many optional parameters — for example, when building immutable objects or configuration payloads.
Why: It avoids messy constructors with too many arguments and improves readability. It also promotes immutability and clean code, often used in DTOs, test data, and chained APIs.
🧱 Structural Patterns
✅ Adapter Pattern
Where to use: Use Adapter when you need to make incompatible interfaces work together — such as integrating a third-party library with your app’s domain model.
Why: Adapter helps bridge the gap between existing code and external APIs or legacy systems. It’s commonly used in enterprise applications to allow clean integration without modifying existing code.
✅ Decorator Pattern
Where to use: Use Decorator when you want to add behavior to objects at runtime without changing their class — for instance, wrapping a service with logging or caching.
Why: Decorator promotes the Open/Closed Principle — open for extension but closed for modification. It allows for flexible functionality layering without altering the base logic.
✅ Proxy Pattern
Where to use: Use Proxy when you want to control access to an object, such as adding security, lazy loading, or remote access control (e.g., in Spring AOP or Hibernate lazy loading).
Why: It acts as a gatekeeper to your core logic, improving performance or enforcing control without changing the original object. It’s frequently used in frameworks for cross-cutting concerns.
🔄 Behavioral Patterns
✅ Observer Pattern
Where to use: Use Observer when multiple objects need to be notified of changes to another object — for example, in event systems, GUIs, or reactive applications.
Why: Observer decouples the subject from its observers, promoting a reactive and modular architecture. It’s widely seen in MVC frameworks and in pub/sub messaging systems.
✅ Strategy Pattern
Where to use: Use Strategy when you need to swap behavior or algorithms at runtime — like sorting logic, payment methods, or discount strategies.
Why: Strategy encapsulates each behavior into its own class, promoting clean separation of concerns. It makes your code more testable and flexible to extend with new behaviors.
✅ Command Pattern
Where to use: Use Command when you want to encapsulate a request as an object, like in undo/redo functionality, task queues, or scheduled job execution.
Why: Command decouples the invoker from the receiver, allowing you to queue, log, or retry operations cleanly. It’s a powerful pattern for task-based architectures or action-driven systems.
✅ Conclusion
Design patterns are not just academic — they are used daily in real-world software systems to write cleaner, modular, and maintainable code. Learning where and why to apply each pattern helps you solve problems more effectively and design applications that scale.
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