The Java & Design Patterns series explains design principles and patterns through runnable Java code: SOLID, the GoF patterns, dependency injection, Repository and Specification with JPA, DDD, Hexagonal Architecture, ...
The Flyweight pattern shares the immutable part of your data across millions of objects to save memory, while Bridge splits two independent axes of variation so the number of classes does not explode...
An online store's shopping cart has single products, bundles, and bundles that contain other bundles. A directory on a server holds files and subdirectories. A menu has items and submenus. All three a...
The Facade pattern puts a simple class in front of a group of complex classes, so a client calls one method instead of orchestrating five or six services in the right order. In a retail system, "place...
The Proxy pattern puts a stand-in object in front of the real one, with the same interface, to control access: intercepting calls, logging, caching, checking permissions, or delaying initialization. I...
The Decorator pattern adds behavior to an object by wrapping it in another object that has the same interface, instead of creating a subclass for every combination of features. You probably use it eve...
A typical e-commerce project often has an old payment gateway: a vendor .jar that takes amounts in the smallest currency unit, returns an integer code instead of a typed result, and throws checked exc...
You have a sample order with the shipping address and product list already configured, and you need thousands of copies with a few fields changed for each customer. Rewriting the whole initialization...
An order has an ID, a customer, a list of products, a quantity, an express-delivery flag, a note and several other optional fields. If you write one constructor per combination, the calling code is ha...
The line new EmailSender() sits deep inside a business class and looks harmless, until the day a customer asks for SMS, then Zalo, and your test suite can't run without an SMTP server. Factory Method...
Every enterprise Java codebase has a few objects that "should exist only once": a configuration holder, a connection pool, a product catalog cache, a logger. The Singleton pattern is the classic way t...
Most enterprise Java systems start out clean and gradually get tangled: one OrderService class calculates prices, writes to the database, and sends email; adding a promotion type means reopening a met...
