Add jep513 module: flexible constructor bodies on JDK 25 and 27

Runnable sources, broken examples and captured transcripts for the ankurm.com JEP 513 article, including the JDK 25 vs 27 compiler wording differences. Co-Authored-By: Claude Sonnet 5 <[email protected]> Claude-Session: https://claude.ai/code/session_01TF9JWFvJSNm6HVzswzZU5a
2026-09-24 10:42:36 +00:00
parent e0cb6e83e2
commit 2ebea58d39
30 changed files with 510 additions and 0 deletions
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+// The classic superclass-constructor trap, and the fix JEP 513 makes possible.
+public class OverridableCall {
+    static class Base {
+        Base() {
+            IO.println("  Base constructor calls describe(): " + describe());
+        }
+        String describe() { return "base"; }
+    }
+
+    // The bug: describe() runs from Base's constructor, before Broken's own field has been assigned.
+    static class Broken extends Base {
+        private final String label;
+        Broken(String label) {
+            super();
+            this.label = label;
+        }
+        @Override String describe() { return "label=" + label; }
+    }
+
+    // The fix: assign the field in the prologue, before super() runs Base's constructor.
+    static class Fixed extends Base {
+        private final String label;
+        Fixed(String label) {
+            this.label = label;   // allowed in the prologue: a plain assignment to a field of THIS class
+            super();
+        }
+        @Override String describe() { return "label=" + label; }
+    }
+
+    public static void main(String[] args) {
+        IO.println("Broken (assign after super()):");
+        new Broken("hello");
+        IO.println("Fixed (assign before super()):");
+        new Fixed("hello");
+    }
+}