Object declaration

There are a few ways to declare singletons in Java. Here is the most common way to define a class that has a private constructor and retrieves instances via a static factory method:

    public class Singleton { 
 
        private Singleton() { 
        } 
 
        private static Singleton instance; 
 
        public static Singleton getInstance() { 
            if (instance == null) { 
                instance = new Singleton(); 
            } 
 
            return instance; 
        } 
    } 

The preceding code works fine for a single thread, but it's not thread-safe, so in some cases two instances of Singleton can be created. There are a few ways to fix it. We can use the synchronized block, presented as follows:

    //synchronized  
    public class Singleton { 
 
        private static Singleton instance = null; 
 
        private Singleton(){ 
        } 
 
        private synchronized static void createInstance() { 
            if (instance == null) { 
            ...