The five SOLID principles in Java 25, each shown as a violating design, the real output that proves it hurts, and the refactored code. Includes a class diagram, one-line interview answers and a runnable file.
Scenario-based Java interview questions for developers with about five years of experience: lost updates, deadlocks, mutable HashMap keys, ThreadLocal leaks, parallel streams, CompletableFuture and production debugging. Eight bugs reproduced in a downloadable Java file with real output, plus 32 answered questions.
50 SQL interview queries with answers for Java developers: Nth highest salary, duplicates, joins, GROUP BY and HAVING, window functions, recursive CTEs and indexes. One sample schema, every query run on PostgreSQL 16 with its real result, plus a downloadable SQL file.
40 scenario-based Kafka interview questions with short answers: ordering, acks and replication, rebalancing, lag, exactly-once, dead-letter topics and share groups. Claims are verified against a real Kafka 4.4.0 broker with downloadable Java files and their actual output.
40 microservices interview questions with short answers and the trap behind each, plus seven patterns (retry, circuit breaker, idempotency, saga, outbox, rate limiting, correlation id) that run for real in a downloadable Java 25 file with its actual output.
A runnable Spring Boot 4.1 microservices project: Spring Cloud Gateway, Eureka, Config Server, product/order/inventory services, a Resilience4j circuit breaker and tracing, with real output.
A complete Spring Boot 4.1 CRUD REST API on a real MySQL 8.4 server: entity, repository, service, controller, validation, ProblemDetail errors, Flyway, with real curl requests and responses.
The same Spring Boot 4.1 app written package-by-layer and package-by-feature, with the public surface counted, the compiler boundary captured and the component-scanning trap shown.
60 Java viva questions and answers covering basics, OOP, exceptions, collections, threads and JDBC, each with a runnable example and its real output.
10 Spring Boot project ideas for a resume with MVP scope, stretch goals, data models and the guides to build them; the cores of three ideas (URL shortener, rate limiter, expense splitter) are real, runnable code.