patterns1SingletonPattern

1package pkg8patterns;2 3/*4 * Singleton (Creational)5 * ----------------------6 * INTENT: ensure a class has exactly one instance and a global access point.7 * UML:  Singleton  - instance: Singleton  + getInstance(): Singleton8 * PROS: controlled single instance; lazy init; saves resources.9 * CONS: global state (hard to test); can hide dependencies; concurrency care needed.10 * REAL-WORLD: Runtime, Logger, configuration, connection pool.11 */12public class patterns1SingletonPattern {13 14    // Best practice: enum singleton (thread-safe, serialization-safe).15    enum Config {16        INSTANCE;17        private int version = 1;18        int getVersion() { return version; }19    }20 21    // Classic lazy + thread-safe via holder idiom (lazy, no locking cost).22    static class Logger {23        private Logger() {}24        private static class Holder { static final Logger INSTANCE = new Logger(); }25        static Logger getInstance() { return Holder.INSTANCE; }26        void log(String m) { System.out.println("[LOG] " + m); }27    }28 29    public static void main(String[] args) {30        System.out.println("enum singleton same? " + (Config.INSTANCE == Config.INSTANCE));31        System.out.println("version = " + Config.INSTANCE.getVersion());32 33        Logger a = Logger.getInstance();34        Logger b = Logger.getInstance();35        System.out.println("holder singleton same? " + (a == b));36        a.log("hello from the one and only logger");37    }38}