Adding Values Within Jtextfield?

I have a supermarket checkout line where i have a list of available products on the left and then a basket on the right with the products in. The products are listed in an array, here is the product class

public class Product {
private String name;
private double weight;
private double price;

[Code] ....

with getters and setters excluded, and the list these are put into

public class productList extends DefaultListModel {
public productList (){
super();
}
public void addProduct (String name, double weight, double price, int code){
super.addElement(new Product(name, weight, price, code));

i have the price for each product to be displayed in a text field with the following code

addBasketItem = new JButton();
getContentPane().add(addBasketItem);
addBasketItem.setText("Add >");

[Code] ....

defaultCheckoutList contains my available items and defaultMainList is the basket, with mainTillPrice being the jtextfield.

This works to get the price however it just replaces each time i make a new entry with the price for the next item, i want a total of the price of all the items i have added, but not sure how.