5 java singleton samples

SingletonDemo.java

public class SingletonDemo {
    public static class Singleton1{
        private static final Singleton1 instance = new Singleton1();
        private Singleton1(){}
        public static Singleton1 getInstance(){
            return instance;
        }
    }

    public static class Singleton2{
        private static Singleton2 instance;
        private Singleton2(){};
        public static synchronized Singleton2 getInstance(){
            if(instance == null){
                instance = new Singleton2();
            }
            return instance;
        }
    }

    public static class Singleton3{
        private static volatile Singleton3 instance;
        private Singleton3(){}
        public static Singleton3 getInstance(){
            if(instance == null){
                synchronized (Singleton3.class){
                    if(instance == null){
                        instance = new Singleton3();
                    }
                }
            }
            return instance;
        }
    }

    public static class Singleton4{
        private static final Singleton4 instance;
        static{
            instance = new Singleton4();
        }
        private Singleton4(){}
        public static Singleton4 getInstance(){
            return instance;
        }
    }

    public static class Singleton5{
        private static class SingletonHolder{
            private static final Singleton5 instance = new Singleton5();
        }
        private Singleton5(){}
        public static Singleton5 getInstance(){
            return SingletonHolder.instance;
        }
    }
}