Build refresh token rotation with token families and reuse detection on Spring Boot 4.1 and Spring Security 7.1. Opaque hashed tokens, a race-proof conditional UPDATE, a grace window, logout everywhere, and the access token that outlives logout, all run and captured.
Build a library management system in Java in three stages: one runnable file, then proper classes, then a Spring Boot 4 REST API with JPA. Includes a real race condition (8 loans for 1 copy), the N+1 query trap, and every output captured from a real run.
Spring Boot 4.1.2 test slices run side by side on one app: what @WebMvcTest, @JsonTest, @RestClientTest and @DataJpaTest each build (35 to 251 bean definitions), the exact errors a missing mock, an unnamed client and @EnableJpaAuditing produce, and the Boot 4 package moves that break every old import. Every figure comes from a committed test run.
What Hibernate 8's EntityAgent is, how it differs from EntityManager, and what it really does on 8.0.0.CR1: no dirty checking, lazy fetch(), bulk inserts, lock modes, programmatic result set mappings, and where H2 and HSQLDB disagree. Every figure comes from a committed test run.
A Java DateTimeFormatter cheat sheet where every output was run on JDK 25: pattern letters, why YYYY prints next year in late December (36 wrong days in 11 years for en_US and en_IN), the U+202F space that breaks AM/PM parsing since JDK 20, SMART vs STRICT parsing of 31 February, ISO constants, and a shared SimpleDateFormat that returns wrong dates.
Spring Data JPA projections, tested on Spring Boot 4.1.2 and Hibernate 7.4: closed and open interface projections, record DTOs, dynamic projections and @Query constructor expressions, with the real SQL Hibernate generated for each, the @Value trap that loads full rows, and three @Query traps that fail quietly.
Learn stack and queue problems in Java from scratch: balanced brackets, RPN, min stack, next greater element, largest rectangle in a histogram, decode string, queue from two stacks and stack from one queue, with dry runs, brute-force cross-checks and operation counts from real JDK 25 runs.
Copying an object does not copy what it points at. A run-it-yourself walk through shallow vs deep copy in Java: why clone() is broken, copy constructors, serialization copies, records and defensive copies, with real output from a 24-test companion module.
Why Java needs both equals() and hashCode(), shown by breaking them: a HashSet holding duplicates, a constant hash costing 2,002,999 comparisons for 2,000 lookups, a mutated key that get() and remove() cannot find, record edge cases, and the JPA entity id trap. Every output is from a JDK 25 run and a committed transcript.
A Java String.format cheat sheet where every output was run on JDK 25: specifier anatomy, padding and alignment, why %.2f can disagree with C, locale traps, the lakh/crore grouping Formatter cannot produce (with a working formatter), the exceptions it throws, and what it costs against +.