123-java-design-patterns

# Java Design and Integration Patterns Guide Java developers in selecting patterns by problem signal, implementation context, and trade-off rather than by pattern name alone. **What is covered in this Skill?** - Classic Java design patterns for application code: creational, structural, and behavioral patterns - REST API patterns for resource design, contracts, idempotency, versioning, and error handling - Kafka and event-driven patterns for event schemas, partitioning, idempotency, retries, outbox, and sagas - Database and persistence patterns for repositories, transactions, aggregates, locking, migrations, and read models - Cross-cutting integration patterns for anti-corruption layers, resilience, observability, and reliable message boundaries **Scope:** Use this skill to explain, review, and implement practical patterns in Java systems. Prefer simple code first; introduce a pattern only when it reduces real complexity, protects a boundary, improves testability, or makes change safer. ## Constraints Pattern guidance must be problem-led, concrete, and safe to apply in Java projects. - **PROBLEM FIRST**: Identify the design pressure before naming or applying a pattern - **NO PATTERN SHOPPING**: Do not add abstractions only because a pattern exists; prefer simple code when variation is not present - **BUILD SAFETY**: For code changes, run `./mvnw compile` or `mvn compile` before refactoring and `./mvnw clean verify` or `mvn clean verify` after changes - **REFERENCE SELEC...