// Eager initialization (thread-safe)public class EagerSingleton {    private static final EagerSingleton instance = new EagerSingleton();        private EagerSingleton() {        // Private constructor prevents instantiation    }        public static EagerSingleton getInstance() {        return instance;    }}// Lazy initialization (NOT thread-safe)public class LazySingleton {    private static LazySingleton instance;        private LazySingleton() {}        public static LazySingleton getInstance() {        if (instance == null) {            instance = new LazySingleton();        }        return instance;    }}// Thread-safe lazy initialization with synchronizedpublic class SynchronizedSingleton {    private static SynchronizedSingleton instance;        private SynchronizedSingleton() {}        public static synchronized SynchronizedSingleton getInstance() {        if (instance == null) {            instance = new SynchronizedSingleton();        }        return instance;    }}// Double-checked locking (thread-safe)public class DoubleCheckedSingleton {    private static volatile DoubleCheckedSingleton instance;        private DoubleCheckedSingleton() {}        public static DoubleCheckedSingleton getInstance() {        if (instance == null) {            synchronized (DoubleCheckedSingleton.class) {                if (instance == null) {                    instance = new DoubleCheckedSingleton();                }            }        }        return instance;    }}// Bill Pugh solution (recommended - thread-safe, lazy)public class BillPughSingleton {    private BillPughSingleton() {}        private static class SingletonHelper {        private static final BillPughSingleton instance = new BillPughSingleton();    }        public static BillPughSingleton getInstance() {        return SingletonHelper.instance;    }}// Enum singleton (simplest, thread-safe, serializable)public enum EnumSingleton {    INSTANCE;        public void doSomething() {        // Singleton methods here    }}

// Eager singletonEagerSingleton eager1 = EagerSingleton.getInstance();EagerSingleton eager2 = EagerSingleton.getInstance();System.out.println(eager1 == eager2); // true - same instance// Lazy singleton (not thread-safe)LazySingleton lazy1 = LazySingleton.getInstance();LazySingleton lazy2 = LazySingleton.getInstance();System.out.println(lazy1 == lazy2); // true - same instance// Thread-safe singletonSynchronizedSingleton sync1 = SynchronizedSingleton.getInstance();SynchronizedSingleton sync2 = SynchronizedSingleton.getInstance();System.out.println(sync1 == sync2); // true// Double-checked singletonDoubleCheckedSingleton dcs1 = DoubleCheckedSingleton.getInstance();DoubleCheckedSingleton dcs2 = DoubleCheckedSingleton.getInstance();System.out.println(dcs1 == dcs2); // true// Bill Pugh singleton (recommended)BillPughSingleton bp1 = BillPughSingleton.getInstance();BillPughSingleton bp2 = BillPughSingleton.getInstance();System.out.println(bp1 == bp2); // true// Enum singleton (simplest)EnumSingleton enum1 = EnumSingleton.INSTANCE;EnumSingleton enum2 = EnumSingleton.INSTANCE;System.out.println(enum1 == enum2); // trueenum1.doSomething();// In multithreaded environmentExecutorService executor = Executors.newFixedThreadPool(10);for (int i = 0; i < 10; i++) {    executor.submit(() -> {        BillPughSingleton instance = BillPughSingleton.getInstance();        System.out.println("Thread: " + Thread.currentThread().getName() + ", Instance: " + instance);    });}executor.shutdown();