package singleton; import java.util.ArrayList; class SingletonEager { private static SingletonEager singletonObject = new SingletonEager(); private SingletonEager() { } public static SingletonEager getInstance() { return singletonObject; } } // Lazy Initialization class SingletonLazy { private static SingletonLazy singleton = null; private SingletonLazy() { } public static SingletonLazy getInstance() { if(singleton == null) { singleton = new SingletonLazy(); } return singleton; } } // Lazy Initialization with Synchronization class SingletonSync { private static SingletonSync singleton = null; private SingletonSync() { } // 1000 threads, T1 public synchronized static SingletonSync getInstance() { // for loop if(singleton == null) { singleton = new SingletonSync(); } return singleton; } } // Lazy Initialization with Synchronization Double Locking public class Singleton { private static Singleton singleton = null; private Singleton() { } // 1000 threads, T1 public static Singleton getInstance() { // for loop, T2 if(singleton == null) { // T1 => singleton1 = new Singleton() // T2 => singleton2 = new Singleton() synchronized (Singleton.class) { if(singleton == null) { singleton = new Singleton(); } } } return singleton; } }