Java 27 and 26: runnable demos and captured output for every JEP, plus version lanes

Co-Authored-By: Claude Sonnet 5 <[email protected]> Claude-Session: https://claude.ai/code/session_01B38FGKKam5SCGgwgduVAh3
2026-09-21 15:08:50 +00:00
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+import java.util.ArrayList;
+import java.util.List;
+import org.openjdk.jol.info.ClassLayout;
+import org.openjdk.jol.info.GraphLayout;
+import org.openjdk.jol.vm.VM;
+
+/**
+ * JEP 534 seen from the inside: how many bytes does each object spend on its header, and what
+ * does that add up to across a million objects?
+ *
+ * <p>Uses JOL 0.17 ({@code org.openjdk.jol:jol-core}). Explained in docs/03-compact-headers.md.
+ * Run by scripts/object-headers.sh, once per JDK.
+ */
+public class HeaderDemo {
+
+    /** Two ints. The smallest object that is not empty. */
+    static class Point {
+        int x;
+        int y;
+    }
+
+    /** A more typical mix: a long, an int, a boolean and a reference. */
+    static class Order {
+        long id;
+        int quantity;
+        boolean paid;
+        String customer;
+    }
+
+    public static void main(String[] args) {
+        System.out.println("java.version = " + System.getProperty("java.version"));
+        System.out.println(VM.current().details());
+
+        section("java.lang.Object");
+        System.out.println(ClassLayout.parseClass(Object.class).toPrintable());
+
+        section("Point { int x; int y; }");
+        System.out.println(ClassLayout.parseClass(Point.class).toPrintable());
+
+        section("Order { long id; int quantity; boolean paid; String customer; }");
+        System.out.println(ClassLayout.parseClass(Order.class).toPrintable());
+
+        section("java.lang.Long and java.lang.Integer (boxed values)");
+        System.out.println(ClassLayout.parseInstance(Long.valueOf(123_456_789L)).toPrintable());
+        System.out.println(ClassLayout.parseInstance(Integer.valueOf(123_456)).toPrintable());
+
+        section("int[3]");
+        System.out.println(ClassLayout.parseInstance(new int[3]).toPrintable());
+
+        section("What it adds up to: one million objects");
+        List<Integer> boxed = new ArrayList<>();
+        List<Long> longs = new ArrayList<>();
+        List<Point> points = new ArrayList<>();
+        for (int i = 0; i < 1_000_000; i++) {
+            boxed.add(1_000 + i);          // above the Integer cache, so every one is a real object
+            longs.add(10_000_000_000L + i);
+            Point p = new Point();
+            p.x = i;
+            p.y = -i;
+            points.add(p);
+        }
+        System.out.printf("1,000,000 boxed Integer values in an ArrayList : %,d bytes%n",
+                GraphLayout.parseInstance(boxed).totalSize());
+        System.out.printf("1,000,000 boxed Long values in an ArrayList    : %,d bytes%n",
+                GraphLayout.parseInstance(longs).totalSize());
+        System.out.printf("1,000,000 Point objects in an ArrayList        : %,d bytes%n",
+                GraphLayout.parseInstance(points).totalSize());
+    }
+
+    static void section(String title) {
+        System.out.println();
+        System.out.println("=== " + title);
+    }
+}
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+import org.openjdk.jol.info.ClassLayout;
+
+/**
+ * JOL 0.17 cannot lay out a record out of the box: it asks sun.misc.Unsafe for field offsets, and
+ * Unsafe refuses for records. -Djol.magicFieldOffset=true switches JOL to a slower workaround.
+ * Run by scripts/object-headers.sh; both outcomes are committed as transcripts.
+ */
+public class RecordProbe {
+    record Coordinates(int lat, int lon) {}
+
+    public static void main(String[] args) {
+        System.out.println(ClassLayout.parseClass(Coordinates.class).toPrintable());
+    }
+}