Sales Opportunity - Summing Up Variables?

I have this small program that creates a sales opportunity and its related products, and then prints out the opportunity name, total quantity and total price

class Opportunity{
String name;
int Amount;
String Stage;
int Product_quantity;

[Code] .....

The code works as expected, and is successfully prints :

Opportunity Name: Sales
Opportunity Amount: 110
Opportunity Product Quantity: 3

That said, I feel the way the OpportunityCreator class sums up the quantities and prices (see the comments on the code) is very inefficient, and would only work if there were 3 or less products. How can I design this so that it sums the quantities and prices of all the Product objects that I create, regardless of how many of them are? I understand probably I'd need to redesign the whole thing but I'd like to get at least an idea of how this can be achieved.

Originally I was thinking of creating an ArrayList and adding all the prices/quantities there but wasn't sure how to do. I also thought of using a loop to loop through the different object references and sum the quantity/price but was also unsure on how to do so?