main.java

public class main{
    public static void main(String[] args) {
        //WORKS
        SingletonDemo s = SingletonDemo.getInstance();
        s.setValue(12);
        //FAILS
        SingletonDemo ss = SingletonDemo.getInstance();
        System.out.println(ss.getValue());
    }
}

class SingletonDemo {
        private static volatile SingletonDemo instance = null;

        private int value;
 
        private SingletonDemo() {       }
 
        public static SingletonDemo getInstance() {
                if (instance == null) {
                        instance = new SingletonDemo ();
                }
                return instance;
        }

        public void setValue(int value){
            this.value = value;
        }

        public int getValue(){
            return this.value;
        }
}