design-patterns

Create a static nested Builder class holding the same fields, make the outer constructor private, and expose a fluent API where each setter returns the builder and build() constructs the object. This avoids telescoping constructors when many parameters are optional.

Use Builder when constructing one complex object with many optional parameters. Use Factory when the exact class to instantiate is determined at runtime, such as selecting a notification sender by type string.

Publish events with ApplicationEventPublisher and consume them with @EventListener annotated methods on Spring components. Adding @Async lets listeners like email senders run without blocking the publisher.

Yes. Define the strategy as a @FunctionalInterface with a single method, then pass lambdas or method references as implementations. This removes the need for separate concrete strategy classes for simple behaviors.

Avoid Singleton abuse, which creates global state and hinders testing; prefer dependency injection. Avoid factories when the type is known at compile time, and keep decorator chains short to remain debuggable.