Removing ArrayList object duplicates and getting sum of value

Question

I have a list of a certain type of Object. I need to remove the duplicates object key field and sum up their values. It's kinda hard to explain so let me show you an example.

Say you have an Item class:

public class Item {

protected String name;

protected int quantity;

public String getName() {
    return name;
   }

public void setName(String name) {
    this.name = name;
   }

public int getQuantity() {
    return quantity;
   }

public void setQuantity(intquantity) {
    this.quantity = quantity;
   }

}

And you have a list of Item's:

List<Item> itemList = new ArrayList<Item>();

Populated as:

 Item item1 = new Item();
 item1.setName("mice");
 item1.setQuantity(20);

 Item item2 = new Item();
 item2.setName("keyboards");
 item2.setQuantity(30);


 Item item3 = new Item();
 item3.setName("monitors");
 item3.setQuantity(4);

 Item item4 = new Item();
 item4.setName("mice");
 item4.setQuantity(15);

 Item item5 = new Item();
 item5.setName("cables");
 item5.setQuantity(50);


 itemList.add(0, item1);
 itemList.add(1, item2);
 itemList.add(2, item3);
 itemList.add(3, item4);
 itemList.add(4, item5);

I need to have an output ArrayList that doesn't have duplicates, and adds up the quantity value.

So essentially, the result should be an arraylist of elements: mice, keyboards, monitors, cables where mice quantity = 35 (the sum), the keyboards quantity = 30, monitors = 4, cables = 50.

Answer 1

Provided you are using Java 8, you can use Collectors#groupingBy:

itemList.stream().collect(
    Collectors.groupingBy(Item::getName, Collectors.summingInt(Item::getQuantity)));

Answer 2

use a HashMap:

HashMap<String, Integer> myItemMap = new HashMap<String,Integer>();
if(myItemMap.containsKey(item)
{
  int currentQty = myItemMap.get(item);
  myItemMap.get(item).setQuantity(qty + currentQty )
}
else
{
  myItemMap.put(item, qty);
}