javademos/jep-tour/src/LazyDemo.java

import java.util.List;
import java.util.Map;
import java.util.Set;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.Executors;
import java.util.concurrent.atomic.AtomicInteger;

/**
 * JEP 531, Lazy Constants (third preview), against the JDK 27 API.
 *
 * <p>A lazy constant is a value that is computed the first time somebody asks for it and never again,
 * even when many threads ask at once. The JVM can then treat it like a {@code final} field. What changed
 * since JDK 26: {@code orElse(...)} and {@code isInitialized()} are gone (see broken/Lazy26.java), and
 * {@code Set.ofLazy(...)} arrived next to {@code List.ofLazy} and {@code Map.ofLazy}.
 * Explained in docs/06-lazy-constants.md. Compile and run with --enable-preview.
 */
public class LazyDemo {

    static final AtomicInteger INITIALISATIONS = new AtomicInteger();

    /** The expensive thing: pretend this reads a file and parses it. */
    record Settings(String region, int poolSize) {}

    static Settings loadSettings() {
        INITIALISATIONS.incrementAndGet();
        System.out.println("  (loading settings on " + Thread.currentThread().getName() + ")");
        return new Settings("ap-south-1", 16);
    }

    static final LazyConstant<Settings> SETTINGS = LazyConstant.of(LazyDemo::loadSettings);

    /** Identity hashes and lambda addresses change on every run; blank them so the transcript is stable. */
    static String tidy(Object o) {
        return o.toString().replaceAll("@[0-9a-f]+", "@...").replaceAll("/0x[0-9a-f]+", "/0x...");
    }

    public static void main(String[] args) throws Exception {
        System.out.println("--- a lazy constant computes once, on first use");
        System.out.println("toString before first get(): " + tidy(SETTINGS));
        Settings first = SETTINGS.get();
        Settings second = SETTINGS.get();
        System.out.println("toString after  first get(): " + tidy(SETTINGS));
        Check.that(first == second, "get() returns the same instance every time");
        Check.that(INITIALISATIONS.get() == 1, "the supplier ran exactly once");

        System.out.println("--- and once even when 64 threads race for it");
        var raced = LazyConstant.of(() -> {
            INITIALISATIONS.incrementAndGet();
            return "computed";
        });
        int before = INITIALISATIONS.get();
        var start = new CountDownLatch(1);
        try (var pool = Executors.newVirtualThreadPerTaskExecutor()) {
            for (int i = 0; i < 64; i++) {
                pool.submit(() -> {
                    start.await();
                    return raced.get();
                });
            }
            start.countDown();
        }
        Check.that(INITIALISATIONS.get() - before == 1, "64 racing threads triggered exactly one initialisation");

        System.out.println("--- List.ofLazy: each element is computed on first access");
        AtomicInteger listComputations = new AtomicInteger();
        List<String> squares = List.ofLazy(5, i -> {
            listComputations.incrementAndGet();
            return "square(" + i + ")=" + (i * i);
        });
        System.out.println("computed so far: " + listComputations.get());
        System.out.println("squares.get(3) = " + squares.get(3));
        System.out.println("squares.get(3) = " + squares.get(3) + "   (second read)");
        Check.that(listComputations.get() == 1, "only element 3 was ever computed");

        System.out.println("--- Map.ofLazy: the keys are fixed, the values are computed on first access");
        AtomicInteger mapComputations = new AtomicInteger();
        Map<String, Integer> lengths = Map.ofLazy(Set.of("alpha", "beta", "gamma"), k -> {
            mapComputations.incrementAndGet();
            return k.length();
        });
        System.out.println("lengths.get(\"gamma\") = " + lengths.get("gamma"));
        Check.that(mapComputations.get() == 1, "only the requested key's value was computed");

        System.out.println("--- Set.ofLazy (new in 27): membership is decided by a predicate, on demand");
        AtomicInteger setComputations = new AtomicInteger();
        Set<Integer> primes = Set.ofLazy(Set.of(2, 3, 4, 5, 6, 7), n -> {
            setComputations.incrementAndGet();
            return n > 1 && java.util.stream.IntStream.rangeClosed(2, (int) Math.sqrt(n)).noneMatch(d -> n % d == 0);
        });
        System.out.println("primes.contains(7) = " + primes.contains(7) + ", primes.contains(6) = " + primes.contains(6));
        Check.that(primes.contains(7) && !primes.contains(6), "the lazy set answers membership using the predicate");
        System.out.println("primes = " + new java.util.TreeSet<>(primes));
    }
}