Binary tree traversal in Java on one numbered nine-node tree: recursive and iterative inorder, preorder and postorder with stack dry-run tables, one-stack postorder, Morris traversal, level order and zigzag with a queue, height, an O(n) versus O(n squared) diameter comparison by counted visits, a real StackOverflowError on a one-million-node skewed tree, and exam questions with answers.
Anthropic's five agentic workflows (prompt chaining, routing, parallelization, orchestrator-workers, evaluator-optimizer) as plain Spring AI ChatClient code, tested offline with a scripted ChatModel that records every prompt, with the failure modes and the call-count cost of each.
JDK 25 already prints a warning when a library calls sun.misc.Unsafe memory-access methods, and JEP 471 plans for them to throw and then disappear. What each --sun-misc-unsafe-memory-access value does on JDK 25, how jdeps, a bytecode scan and JFR find the callers in real jars (Netty, Guava, Kryo, Hadoop, Cassandra, Protobuf and more), and CAS, off-heap and byte[] code migrated to VarHandle and the FFM API with a before/after equivalence test.
A tiny Spring Boot 4.1.2 app with a deliberate 160-requests-per-second limit, load tested with Gatling 3.16.0's Java DSL: scenarios, feeders and checks, open versus closed injection (and why a closed model hides the overload), an incrementUsersPerSec staircase that finds the knee between 150 and 180 users per second, global assertions that fail the Maven build, and what the Gatling report does and does not contain. The same test is then run in k6 v2.3.0 on the same app and compared.
Retention, targets and meta-annotations, runtime reflection versus compile-time processing, and a complete @GenerateBuilder annotation processor: Filer, Messager, compile errors with real javac output, in-memory tests with javax.tools, and the JDK 23+ trap where a processor on the classpath silently stops running (reproduced on JDK 25).
Which Spring Boot startup hook should you use? A beginner-first comparison on Spring Boot 4.1.2 of CommandLineRunner, ApplicationRunner, an ApplicationReadyEvent listener and SmartLifecycle, with the exact order they run in, how @Order sorts both runner kinds in one list (and why unordered ties come out differently on different JVMs), what happens when a runner or a ready-event listener throws, how a blocking runner holds readiness at 503 while the port is already open, and what tests run. Every figure comes from a companion project whose test suite writes the transcripts.
A beginner-first walk through MapStruct on Spring Boot 4.1.2 with Java records as DTOs: how the generated mapper works, nested and collection mapping, null-value strategies, updating existing objects, and what @AfterMapping, @BeforeMapping and @ObjectFactory really do when the target is an immutable record. Every figure comes out of a companion project whose 19 tests write the transcripts, plus the compiler's own error messages and an honestly labelled, indicative comparison with hand-written code and Dozer. Includes surprises: @MappingTarget on a record compiles to an empty method, and JDK 23+ no longer runs a processor that is only on the classpath.
Hibernate 7.4.14 primary keys, measured: IDENTITY turns JDBC batching off silently (10,000 inserts became 10,000 single executes), a pooled SEQUENCE needed 400 round trips, and a sequence whose INCREMENT BY disagrees with allocationSize produced negative and duplicate ids. On UUIDs, the common premise is half wrong: Style.TIME is a version-1-flagged id with a constant IP/JVM half, while real RFC 9562 version 7 ships as Style.VERSION_7 from Hibernate 7.0.0. Includes order_inserts, AUTO across six dialects, an H2 index-size comparison (PostgreSQL could not be run), and the Spring Data SELECT-before-INSERT on assigned ids.
A servlet Filter, a HandlerInterceptor and an @Aspect all run code around a request, but not at the same point. A traced Spring Boot 4.1 app on Tomcat shows the real order, what each can see, and where each one silently fails.
One invoice built three ways in Spring Boot 4.1: OpenPDF 3.0.5, iText Core 9.8.0 (AGPL) and Flying Saucer 10.7.0 rendering a Thymeleaf template. Checks which one shapes Marathi and Hindi correctly (none by default), the OpenPDF switch that does, streaming vs buffered downloads, font subsetting and timings.