Add all 23 GoF design pattern implementations

01-creational/singleton/Main.java

@@ -0,0 +1,36 @@
+package singleton;
+
+public class Main {
+    public static void main(String[] args) {
+
+        System.out.println("--- Implementation 1: Eager ---");
+        AppConfigEager eager1 = AppConfigEager.getInstance();
+        AppConfigEager eager2 = AppConfigEager.getInstance();
+        System.out.println("Same instance? " + (eager1 == eager2));
+        System.out.println("db.url = " + eager1.getProperty("db.url"));
+
+        System.out.println("\n--- Implementation 2: Naive Lazy ---");
+        AppConfigNaiveLazy lazy1 = AppConfigNaiveLazy.getInstance();
+        AppConfigNaiveLazy lazy2 = AppConfigNaiveLazy.getInstance();
+        System.out.println("Same instance? " + (lazy1 == lazy2));
+
+        System.out.println("\n--- Implementation 3: Synchronized Method ---");
+        AppConfigSynchronized sync1 = AppConfigSynchronized.getInstance();
+        AppConfigSynchronized sync2 = AppConfigSynchronized.getInstance();
+        System.out.println("Same instance? " + (sync1 == sync2));
+
+        System.out.println("\n--- Implementation 4: Double-Checked Locking ---");
+        AppConfigDCL dcl1 = AppConfigDCL.getInstance();
+        AppConfigDCL dcl2 = AppConfigDCL.getInstance();
+        System.out.println("Same instance? " + (dcl1 == dcl2));
+
+        System.out.println("\n--- Implementation 5: Holder Idiom ---");
+        AppConfigHolder holder1 = AppConfigHolder.getInstance();
+        AppConfigHolder holder2 = AppConfigHolder.getInstance();
+        System.out.println("Same instance? " + (holder1 == holder2));
+
+        System.out.println("\n--- Implementation 6: Enum ---");
+        AppConfigEnum enumConfig = AppConfigEnum.INSTANCE;
+        System.out.println("db.url = " + enumConfig.getProperty("db.url"));
+    }
+}