Sync Singleton to Java 25: split 6 implementations into separate files, consolidated Main.java demo

01-creational/singleton/AppConfigNaiveLazy.java

@@ -0,0 +1,16 @@
+package singleton;
+
+public class AppConfigNaiveLazy {
+
+    private static AppConfigNaiveLazy instance; // null until first request
+
+    private AppConfigNaiveLazy() { /* load config */ }
+
+    // BROKEN IN MULTITHREADED CODE — see explanation below
+    public static AppConfigNaiveLazy getInstance() {
+        if (instance == null) {                    // Thread A: checks, sees null
+            instance = new AppConfigNaiveLazy();    // Thread B: also sees null, also enters here
+        }                                           // Both threads create a new AppConfigNaiveLazy!
+        return instance;
+    }
+}