public void addProduct(T t) {
        if (t.getClass().isInstance(ComputerPart.class)) {
            products.add(t);
    }
        if (t.getClass().isInstance(Service.class)) {
            products.add(t);
    }
        if (t.getClass().isInstance(Peripheral.class)) {
            products.add(t);
        }
        else System.out.println("Not the right type of object.");
   }

ComputerPart c;
    c = new ComputerPart(12);

    ComputerOrder<Product> g3 = new ComputerOrder<>();
    g3.addProduct(c);
    g3.print();

}