versions6Java21Features

1package pkg2versions;2 3/*4 * versions6Java21Features.java  (2023, LTS) -- the target version for this project5 * ----------------------------------------------------------------------6 * FEATURES & WHY:7 *  - Virtual Threads (JEP 444)     : cheap threads for massive concurrent blocking I/O.8 *  - Pattern matching for switch    : exhaustive, type-based branching (final).9 *  - Record patterns (JEP 440)      : deconstruct records in switch/instanceof.10 *  - Sequenced collections (JEP 431): getFirst/getLast/reversed on ordered collections.11 *  - String templates, structured concurrency, and scoped values were previews12 *    (not shown here). This file requires Java 21. It is not a Java 19 program.13 */14import java.util.*;15 16public class versions6Java21Features {17 18    sealed interface Shape permits Circle, Rect {}19    record Circle(double r) implements Shape {}20    record Rect(double w, double h) implements Shape {}21 22    static String describe(Object o) {23        // Record patterns + guards + exhaustive switch24        return switch (o) {25            case Circle(double r) when r > 10 -> "big circle r=" + r;26            case Circle(double r)             -> "circle r=" + r;27            case Rect(double w, double h)     -> "rect " + w + "x" + h;28            case null                          -> "null";29            default                            -> "other";30        };31    }32 33    public static void main(String[] args) throws InterruptedException {34        // Record patterns35        System.out.println(describe(new Circle(3)));36        System.out.println(describe(new Circle(20)));37        System.out.println(describe(new Rect(2, 5)));38        System.out.println(describe(null));39 40        // Sequenced collections (Java 21)41        SequencedCollection<Integer> seq = new ArrayList<>(List.of(1, 2, 3, 4));42        System.out.println("first=" + seq.getFirst() + " last=" + seq.getLast());43        System.out.println("reversed=" + seq.reversed());44 45        LinkedHashMap<String, Integer> lhm = new LinkedHashMap<>();46        lhm.put("a", 1); lhm.put("b", 2); lhm.put("c", 3);47        System.out.println("firstEntry=" + lhm.firstEntry() + " lastEntry=" + lhm.lastEntry());48 49        // Virtual threads: launch many cheap threads50        long start = System.currentTimeMillis();51        List<Thread> threads = new ArrayList<>();52        for (int i = 0; i < 1000; i++) {53            int id = i;54            Thread t = Thread.ofVirtual().start(() -> {55                try { Thread.sleep(10); } catch (InterruptedException ignored) {}56            });57            threads.add(t);58        }59        for (Thread t : threads) t.join();60        System.out.println("1000 virtual threads finished in " + (System.currentTimeMillis() - start) + "ms");61    }62}