How to add two object together

You should use a numeric data type for arithmetic operations (not Object). With out seeing the code in add() my recommendation is to store the total price in a double primitive.

     double price =0;

     for(int x = 0; x < rprice.size(); x++) {
    //you may need to cast/convert  here
        price += (double)rprice.get(x);

    //what does this do????
        Object numResult = add(totalPrice, price);

        pricelbl.setText(price);

    }