public class SingletonTest {
    public static void main(String[] args) {
        FirstSingleton.INSTANCE.hello();
        //output: Hello the first singleton!

        SecondSingleton.getInstance().hello();
        //output: Hello the second singleton!

        ThirdSingleton.INSTANCE.hello();
        //output: Hello the third singleton!
    }
}

class FirstSingleton {
    public static final FirstSingleton INSTANCE = new FirstSingleton();

    private FirstSingleton() {
        //защита от рефлексии
        if (INSTANCE != null) {
            throw new InstanceAlreadyExistException();
        }
    }

    public void hello() {
        System.out.println("Hello the first singleton!");
    }
}

class SecondSingleton {
    private static final SecondSingleton INSTANCE = new SecondSingleton();

    public static SecondSingleton getInstance() {
        return INSTANCE;
    }

    private SecondSingleton() {
        //защита от рефлексии
        if (INSTANCE != null) {
            throw new InstanceAlreadyExistException();
        }
    }

    public void hello() {
        System.out.println("Hello the second singleton!");
    }
}

enum ThirdSingleton {
    INSTANCE;

    public void hello() {
        System.out.println("Hello the third singleton!");
    }
}

class InstanceAlreadyExistException extends RuntimeException {

}