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

01-creational/singleton/AppConfigEager.java

@@ -0,0 +1,24 @@
+package singleton;
+
+public class AppConfigEager {
+
+    // The JVM creates this instance when the class is first loaded.
+    // static guarantees it belongs to the class, not any instance.
+    // final guarantees it can never be reassigned.
+    private static final AppConfigEager INSTANCE = new AppConfigEager();
+
+    // Private constructor: prevents any other class from calling new AppConfigEager()
+    private AppConfigEager() {
+        System.out.println("Reading configuration from file...");
+        // load properties, set up values, etc.
+    }
+
+    // The only way to get the instance
+    public static AppConfigEager getInstance() {
+        return INSTANCE;
+    }
+
+    public String getProperty(String key) {
+        return System.getProperty(key, "(not set)");
+    }
+}