Java 21

This one gets Claude up to speed on Java 21's three headline features: records for immutable data carriers, sealed types with exhaustive pattern matching, and virtual threads for scalable blocking I/O. You'd reach for it when building modern Java services where you want DTOs validated in compact constructors, closed hierarchies that the compiler can verify, or need to handle thousands of concurrent blocking calls without spinning up platform threads. The examples are clean and show real validation patterns, not toy code. Honestly, if you're still on Java 8 or 11 this will feel like a different language, and that's the point. The anti-patterns section usefully calls out the old habits to drop.