java-design-patterns

Create a static nested Builder class with required fields in its constructor and fluent setter methods for optional fields, ending with a build() method. With Lombok, the @Builder annotation generates this automatically, using @Builder.Default for default values.

Use Factory when you need to create objects without specifying the exact class at the call site. Use Strategy when you have multiple algorithms for the same operation and need to swap them at runtime, such as switching payment methods.

Publish events using ApplicationEventPublisher and create listener methods annotated with @EventListener in Spring components. Add @Async to listeners for asynchronous event processing instead of manually maintaining observer lists.

Singletons are discouraged because they create global state and hidden dependencies that complicate testing. In Spring, prefer @Component beans, which are singleton-scoped by default and managed through dependency injection.

Deep decorator chains become hard to debug and reason about since behavior is spread across many wrapper objects. Keep chains short and consider composition or alternative designs when stacking many decorators.