【Java】JDK 27 逆引き — LazyConstant・プリミティブ型パターン

# プレビューはコンパイルと実行の両方で有効にする
javac --release 27 --enable-preview Main.java
java --enable-preview Main

// 遅延初期化: 初回の get() だけが計算する
LazyConstant<String> value = LazyConstant.of(() -> "computed");

// プリミティブ型のパターン: default を型付きの case に置き換える
switch (status) { case int i when i >= 100 -> "大きい"; default -> "その他"; }

// 構造化並行性: サブタスクはスコープを抜ける前に必ず合流する
try (var scope = StructuredTaskScope.<String>open()) { }

int[] calls = { 0 };
var lazy = java.lang.LazyConstant.of(() -> {
    calls[0]++;
    return "computed#" + calls[0];
});
System.out.println("LazyConstant 1回目 = " + lazy.get());
System.out.println("LazyConstant 2回目 = " + lazy.get() + " / ラムダの実行回数 = " + calls[0]);

// 実行結果
// LazyConstant 1回目 = computed#1
// LazyConstant 2回目 = computed#1 / ラムダの実行回数 = 1

List<String> lazyList = List.ofLazy(3, i -> "item" + (i * i));
System.out.println("List.ofLazy = " + lazyList);

Map<String, String> lazyMap = Map.ofLazy(Set.of("a", "b"), String::toUpperCase);
System.out.println("Map.ofLazy = " + lazyMap.get("a") + "," + lazyMap.get("b"));

Set<Option> lazySet = Set.ofLazy(EnumSet.allOf(Option.class), o -> o != Option.DRY_RUN);
System.out.println("Set.ofLazy contains(DRY_RUN)=" + lazySet.contains(Option.DRY_RUN)
        + " size=" + lazySet.size());

// 実行結果
// List.ofLazy = [item0, item1, item4]
// Map.ofLazy = A,B
// Set.ofLazy contains(DRY_RUN)=false size=2

static String describe(int status) {
    return switch (status) {
        case 0 -> "ゼロ";
        case int i when i >= 100 -> "大きい(" + i + ")";
        case int i -> "その他(" + i + ")";
    };
}

System.out.println("switch(int) = " + describe(0) + " / " + describe(150) + " / " + describe(7));

// 実行結果
// switch(int) = ゼロ / 大きい(150) / その他(7)

record JsonNumber(double value) {}

Object boxed = 300;
if (boxed instanceof int i) {
    System.out.println("instanceof int = " + i);
}

Object json = new JsonNumber(30.0);
if (json instanceof JsonNumber(int age)) {
    System.out.println("record パターンで double -> int = " + age);
}

// 実行結果
// instanceof int = 300
// record パターンで double -> int = 30

try (var scope = StructuredTaskScope.<String>open()) {
    var user = scope.fork(() -> "user-7");
    var order = scope.fork(() -> "order-42");
    scope.join();
    System.out.println("StructuredTaskScope join = " + user.get() + " / " + order.get());
}

// 実行結果
// StructuredTaskScope join = user-7 / order-42

try (var scope = StructuredTaskScope.<String, List<String>, java.util.concurrent.ExecutionException>open(
        StructuredTaskScope.Joiner.allSuccessfulOrThrow(),
        cfg -> cfg.withTimeout(Duration.ofMillis(200)))) {
    scope.fork(() -> "fast");
    scope.fork(() -> "faster");
    List<String> results = scope.join();
    System.out.println("timeout 付き join = " + results);
}

// 実行結果
// timeout 付き join = [fast, faster]

java -XX:+PrintFlagsFinal -version | grep UseCompactObjectHeaders

# 実行結果（JDK 27 / JDK 25 の比較）
# JDK 27: bool UseCompactObjectHeaders = true  {product lp64_product} {default}
# JDK 25: bool UseCompactObjectHeaders = false {product lp64_product} {default}

java -XX:+PrintFlagsFinal -version | grep UseG1GC

# 実行結果
# bool UseG1GC = true {product} {ergonomic}

String[] groups = SSLContext.getDefault().getSupportedSSLParameters().getNamedGroups();
System.out.println("TLS named groups = " + String.join(", ", groups)
        + " (" + groups.length + " 個)");

// 実行結果
// TLS named groups = X25519MLKEM768, x25519, secp256r1, secp384r1, secp521r1, x448, ffdhe2048, ffdhe3072, ffdhe4096 (9 個)

var kp = KeyPairGenerator.getInstance("RSA");
kp.initialize(2048);
var pair = kp.generateKeyPair();
AsymmetricKey pub = pair.getPublic();
String pem = PEMEncoder.of().encodeToString(pub);
System.out.println("PEM 1 行目 = " + pem.lines().findFirst().orElse("?"));

// 実行結果
// PEM 1 行目 = -----BEGIN PUBLIC KEY-----