class Smartphone {
}

class Pager {
}

class TabletPC {
}

class MobileDevice<T> {
    private static int count = 0;

    MobileDevice() {
        count++;
        System.out.println(count);
    }
    // ...
}

public class Main {
    public static void main(String[] args) {
        MobileDevice<Smartphone> phone = new MobileDevice<>();
        MobileDevice<Pager> pager = new MobileDevice<>();
        MobileDevice<TabletPC> pc = new MobileDevice<>();
    }
}