How to achieve "lstItemXY" from "listItemArray"?

Question

I have an lstItemArry as shown below example.

  ArrayList lstItemArry= {id  productid  qty
                          1   175        1
                          2   175        1
                          3   182        1
                         }

Logic Implemented

    for (int i = 0; i < lstItemArry.size(); i++) {

        PieDataChart json_data = lstItemArry.get(i);

        for (int j = 0; j < lstItemArry.size(); j++) {

            PieDataXYChart json_xy = new PieDataXYChart();

            if (json_data.getProduct_id() == lstItemArry.get(j).getProduct_id()) {
                json_xy.setProduct_id(
                        json_data.getProduct_id(
                                json_xy.setQuantity(json_data.getQuantity() + lstItemArry.get(j).getQuantity()
                                )
                        )
                );

                lstItemXY.add(json_xy);
            }
        }
    }

    //print all element
    for (PieDataXYChart element : lstItemXY) {
        System.out.println("Number = " + element);
    }

my OutPut received "lstItemXY":

 product_id 182  quantity 2
 product_id 175  quantity 2
 product_id 175  quantity 2   
 product_id 175  quantity 2
 product_id 175  quantity 2

But I wanted my "lstItemXY" to received like below:

 product_id 182  quantity 1
 product_id 175  quantity 2