Cheat sheets
Dense, printable one-pagers for last-minute revision.
Collections & Complexity
Java collection implementations, Big-O costs, selection rules, fail-fast behaviour, ConcurrentHashMap and immutable factories.
Concurrency
Thread lifecycle, JMM primitives, locks, executors, CompletableFuture, virtual threads and a deadlock checklist.
Modern Java (8 to 21)
Language and API features by Java version, with syntax, gotchas and stream recipes.
Spring Essentials
Core Spring Boot 3.x annotations, bean scopes and lifecycle, auto-configuration, profiles, properties and REST/exception-handling recipes.
JPA & Hibernate Pitfalls
Entity states, fetching, N+1 fixes, flushing, optimistic locking, cascades, equals/hashCode, batching and LazyInitializationException.
SQL, Indexing & Isolation
Index types and rules, composite index ordering, reading EXPLAIN, isolation levels versus anomalies, locking reads and slow-query causes.
DevOps Commands
Maven, Git, Docker, kubectl, JDK diagnostic tools, Actuator endpoints, a multi-stage Dockerfile and JVM container flags.
System Design Numbers & Patterns
Latency numbers, capacity formulas, CAP and consistency models, caching, load balancing, DB scaling, messaging semantics, resilience patterns and an answer framework.