JDK 21 enters ramp-down phase 1, finalizing a landmark set of features. Virtual threads are production-ready, enabling high-concurrency web apps. Sequenced collections add first/last access and reversed views to Java's collection hierarchy. Generational ZGC improves memory and CPU efficiency with early benchmarks showing up to 4x throughput gains. Pattern matching for switch and record patterns are both finalized, completing the core pattern matching foundation. On the preview side, structured concurrency graduates from incubation, the Foreign Function & Memory API continues previewing with API refinements, and three new previews debut: unnamed classes and instance main (simpler entry points for beginners), unnamed variables and patterns (cleaner deconstruction with underscore), and string templates (safe, readable string interpolation with domain-specific processors).
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