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)");
+    }
+}