Add jdk28-preview module: JDK 28 early-access build, value classes, strict init flag, PEM, JSON incubator

Co-Authored-By: Claude Sonnet 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01Gth99spB1CzQmBFofXmU4Z

jdk28-preview/src/IntegerIdentity.java

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+/** Compiles on every JDK. Does == on two boxed 1000s say true? */
+public class IntegerIdentity {
+	public static void main(String[] args) {
+		Integer a = 1000, b = 1000;
+		System.out.println("Integer 1000 == Integer 1000: " + (a == b));
+	}
+}

jdk28-preview/src/JsonDemo.java

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+import jdk.incubator.json.*;
+
+/** JEP 540: the incubating JSON API in module jdk.incubator.json. Needs --add-modules jdk.incubator.json. */
+public class JsonDemo {
+	public static void main(String[] args) {
+		JsonValue doc = Json.parse("{\"name\":\"Ada\",\"langs\":[\"en\",\"fr\"],\"age\":36}");
+		System.out.println("name = " + doc.get("name").asString());
+		System.out.println("second language = " + doc.get("langs").get(1).asString());
+		System.out.println("age + 1 = " + (doc.get("age").asInt() + 1));
+		System.out.println("missing key -> " + doc.tryGet("email"));
+		System.out.println(Json.toDisplayString(doc, "  "));
+	}
+}

jdk28-preview/src/PemRoundTrip.java

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+import java.security.*;
+import java.security.spec.ECGenParameterSpec;
+
+/** JEP 542: PEMEncoder / PEMDecoder. Preview in 25; does it still need --enable-preview on 28? */
+public class PemRoundTrip {
+	public static void main(String[] args) throws Exception {
+		KeyPairGenerator g = KeyPairGenerator.getInstance("EC");
+		g.initialize(new ECGenParameterSpec("secp256r1"));
+		KeyPair kp = g.generateKeyPair();
+		String pem = PEMEncoder.of().encodeToString(kp.getPublic());
+		System.out.println(pem.lines().findFirst().get());
+		PublicKey back = PEMDecoder.of().decode(pem, PublicKey.class);
+		System.out.println("round trip equal: " + back.equals(kp.getPublic()));
+	}
+}

jdk28-preview/src/StrictFieldProbe.java

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+/** JEP 539 (preview in JDK 28): a JVM field flag. javac sets it on value-class fields; an identity class is the control. */
+public class StrictFieldProbe {
+	static value class Money { final long cents; Money(long c) { cents = c; } }
+	static class Legacy { final long cents; Legacy(long c) { cents = c; } }
+	public static void main(String[] args) { new Money(1); new Legacy(1); }
+}

jdk28-preview/src/ValueClasses.java

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+/** JEP 401 (preview in JDK 28): value classes and value records give up identity, so == compares state. */
+public class ValueClasses {
+	value record Point(int x, int y) {}
+
+	public static void main(String[] args) {
+		Point p = new Point(1, 2), q = new Point(1, 2);
+		System.out.println("p == q: " + (p == q) + ", equals: " + p.equals(q));
+		System.out.println("isValue: " + p.getClass().isValue());
+		Object o = p;
+		try {
+			synchronized (o) { System.out.println("locked"); }
+		} catch (Throwable t) {
+			System.out.println("synchronized on a value object: " + t);
+		}
+	}
+}