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

01-creational/singleton/AppConfigHolder.java

@@ -0,0 +1,19 @@
+package singleton;
+
+public class AppConfigHolder {
+
+    private AppConfigHolder() { /* load config */ }
+
+    // The JVM loads Holder only when getInstance() is first called.
+    // Class loading is inherently thread-safe — the JVM ensures it happens once.
+    // No synchronized, no volatile, no locks — just the JVM class loading guarantee.
+    private static class Holder {
+        static final AppConfigHolder INSTANCE = new AppConfigHolder();
+    }
+
+    public static AppConfigHolder getInstance() {
+        // This reference triggers Holder to load (if it hasn't already).
+        // After first call, Holder is already loaded and this is just a field read.
+        return Holder.INSTANCE;
+    }
+}