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.lang.reflect.Field;
+
+/**
+ * JEP 500 (Java 26): "Prepare to Make Final Mean Final". Deep reflection on a final field
+ * (Field.setAccessible(true) followed by Field.setInt) used to work silently. On 26 it works but
+ * warns once; a future release will deny it unless you opt in.
+ *
+ * The same file is run five ways by scripts/recap26.sh; the transcript is docs/output/70-final-field-mutation.txt.
+ * Chapter: docs/11-recap-26.md
+ */
+public class FinalFieldMutation {
+    static class Config {
+        private final int port = 8080;
+        int port() { return port; }
+    }
+
+    public static void main(String[] args) throws Exception {
+        Config c = new Config();
+        Field f = Config.class.getDeclaredField("port");
+        f.setAccessible(true);
+        try {
+            f.setInt(c, 9090);
+        } catch (IllegalAccessException e) {
+            System.out.println("setInt refused : " + e.getMessage());
+            return;
+        }
+        // The field reads 9090 but port() still says 8080: 'final int port = 8080' is a constant variable, so javac
+        // copied the literal 8080 into port() at compile time. Nothing you write to the field can reach that copy.
+        // That silent disagreement is the reason the platform wants this to stop, not a hypothetical.
+        System.out.println("field reads    : " + f.getInt(c));
+        System.out.println("port() returns : " + c.port());
+    }
+}