JEP 431's SequencedCollection, SequencedSet and SequencedMap give List, Deque, LinkedHashSet, LinkedHashMap and TreeMap a shared getFirst/getLast/reversed() contract. This replaces the stream.reduce((a,b)->b) and skip(size-1) tricks people use to find the last element of a Set, with real javap-verified interface output and a working demo of why reversed() is a live view, not a copy.
Forty Java Collections interview questions grouped from fundamentals to failure modes, verified against real JDK 25 source and real runs: HashMap bucket layout and treeification reproduced via reflection, a real ConcurrentModificationException alongside the well-known case where it silently does not fire, and the equals/hashCode contract broken and fixed in code.
Real JMH numbers for add, get, iterate and remove on ArrayList and LinkedList, why the gap is bigger than Big-O suggests, the one case LinkedList actually wins, and why ArrayDeque is usually the right answer for stack and queue code instead of either.
JEP 517 added HTTP/3 to java.net.http in Java 26 as an opt-in. What the opt-in looks like, how the three H3_DISCOVERY modes behave, what fallback costs in seconds, why proxies silently downgrade you, and how HTTP/1.1, HTTP/2 and HTTP/3 compare under simulated packet loss, all measured on a local server with captured transcripts.
Stream a 5 GB file at a 64 MB heap with GC evidence, then see the failure modes: readAllLines/readString OOM, unclosed Files.lines and Files.walk leaking file handles, WatchService event folding and the 512-event overflow cliff on Linux, and memory-mapped files. All measured on JDK 25 with a runnable companion module.
The shortest correct way to sort a Java HashMap by value or key (stream into a LinkedHashMap), plus the traps: a TreeMap with a value comparator silently drops tied entries, top-N with a PriorityQueue measured against sort-and-limit in JMH, and JDK 21+ sequenced collections (reversed, firstEntry).
What the string pool and intern() really do, what + compiles to on JDK 25 (invokedynamic, not StringBuilder), a JMH benchmark showing + is fine in one expression but quadratic in a loop, the four text-block rules, and what JEPs 430, 459 and 465 actually say about why string templates never shipped.
Why ObjectInputStream.readObject() on untrusted bytes is dangerous, shown with harmless stand-ins: serialVersionUID drift, code that runs before your cast, and a 37-byte stream that asks for 1 GB. Then the JDK's answer (JEP 290 filters, JEP 415 filter factories), records, and a measured size and speed comparison with Jackson 3 and Protobuf, all from a runnable companion repo.
A beginner-first guide to java.util.regex: Pattern and Matcher, named groups, flags, lookarounds and replaceAll with a lambda, then the failure mode. Measured on JDK 25: which patterns still blow up exponentially, what the JDK already defuses, and fixes that work (rewriting, possessive quantifiers, atomic groups, a CharSequence timeout, input limits).
How Java exceptions really work, measured on JDK 25: cause chaining, checked vs unchecked, try-with-resources suppressed exceptions, JMH numbers for stack-trace capture (with and without writableStackTrace), and why HotSpot sometimes removes the trace for you.