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

README.md

@@ -158,6 +158,26 @@ javac 01-creational/prototype/*.java -d out/prototype
 java -cp out/prototype prototype.Main
 ```
 
+### Singleton (`01-creational/singleton/`)
+
+| Post Section | File(s) |
+|---|---|
+| Implementation 1 — Eager Initialization | `AppConfigEager.java` |
+| Implementation 2 — Naïve Lazy Initialization | `AppConfigNaiveLazy.java` |
+| Implementation 3 — Synchronized Method | `AppConfigSynchronized.java` |
+| Implementation 4 — Double-Checked Locking | `AppConfigDCL.java` |
+| Implementation 5 — Initialization-on-Demand Holder | `AppConfigHolder.java` |
+| Implementation 6 — Enum Singleton | `AppConfigEnum.java` |
+| Running All Six Implementations Together | `Main.java` |
+
+Each implementation is renamed to its own class (e.g. `AppConfigEager`, `AppConfigHolder`) so all six can be compiled together in one package and compared directly. `Main.java` exercises all six in one run. The "Breaking via Reflection," "Breaking via Serialization," and Spring `@Component` snippets are illustrative only — they are not part of this repository's runnable example.
+
+Run it:
+```bash
+javac 01-creational/singleton/*.java -d out/singleton
+java -cp out/singleton singleton.Main
+```
+
 ## Reference
 
 - *Design Patterns: Elements of Reusable Object-Oriented Software* — Gamma, Helm, Johnson, Vlissides