Sync Factory Method, Abstract Factory, Builder, Prototype to Java 25; fix post/code mismatches

01-creational/singleton/AppConfig.java

@@ -0,0 +1,19 @@
+package singleton;
+public class AppConfig {
+
+    private AppConfig() {
+        System.out.println("Reading configuration from file...");
+    }
+
+    private static class Holder {
+        static final AppConfig INSTANCE = new AppConfig();
+    }
+
+    public static AppConfig getInstance() {
+        return Holder.INSTANCE;
+    }
+
+    public String getProperty(String key) {
+        return System.getProperty(key, "(not set)");
+    }
+}

01-creational/singleton/DatabaseConnection.java

@@ -1,34 +0,0 @@
-package singleton;
-
-/**
- * Thread-safe Singleton using double-checked locking.
- */
-public class DatabaseConnection {
-    private static volatile DatabaseConnection instance;
-    private final String url;
-    private int queryCount = 0;
-
-    private DatabaseConnection() {
-        this.url = "jdbc:mysql://localhost:3306/myapp";
-        System.out.println("Establishing connection to " + url);
-    }
-
-    public static DatabaseConnection getInstance() {
-        if (instance == null) {
-            synchronized (DatabaseConnection.class) {
-                if (instance == null) {
-                    instance = new DatabaseConnection();
-                }
-            }
-        }
-        return instance;
-    }
-
-    public String executeQuery(String sql) {
-        queryCount++;
-        return "Result #" + queryCount + " for: " + sql;
-    }
-
-    public int getQueryCount() { return queryCount; }
-    public String getUrl()     { return url; }
-}

01-creational/singleton/Main.java

@@ -1,19 +1,9 @@
 package singleton;
-
 public class Main {
     public static void main(String[] args) {
-        System.out.println("=== Singleton Pattern Demo ===\n");
-
-        DatabaseConnection c1 = DatabaseConnection.getInstance();
-        DatabaseConnection c2 = DatabaseConnection.getInstance();
-        DatabaseConnection c3 = DatabaseConnection.getInstance();
-
-        System.out.println("\nAll three references same instance? " + (c1 == c2 && c2 == c3));
-
-        System.out.println(c1.executeQuery("SELECT * FROM users"));
-        System.out.println(c2.executeQuery("SELECT * FROM orders"));
-        System.out.println(c3.executeQuery("SELECT COUNT(*) FROM products"));
-
-        System.out.println("\nTotal queries: " + c1.getQueryCount());
+        AppConfig config1 = AppConfig.getInstance();
+        AppConfig config2 = AppConfig.getInstance();
+        System.out.println("Same instance? " + (config1 == config2)); // true
+        System.out.println("db.url = " + config1.getProperty("db.url"));
     }
 }