Add all 23 GoF design pattern implementations

01-creational/singleton/AppConfigSynchronized.java

@@ -0,0 +1,16 @@
+package singleton;
+
+public class AppConfigSynchronized {
+
+    private static AppConfigSynchronized instance;
+
+    private AppConfigSynchronized() { /* load config */ }
+
+    // synchronized: only one thread can execute this method at a time
+    public static synchronized AppConfigSynchronized getInstance() {
+        if (instance == null) {
+            instance = new AppConfigSynchronized(); // safe — no other thread can be here
+        }
+        return instance;
+    }
+}