Java 8 separate list elements by class

Question

given I have a list of items that all inherit from the same base class:

class Item {
    protected String name;

    public Item(String name) {

        this.name = name;
  }

  getName() {

      return name;
  }
}

class ItemA extends Item {...}
class ItemB extends Item {...}

List<Item> itemList = Arrays.asList(new ItemA("itemA"), new ItemB("itemB"));

In my case I have no control over how these classes are implemented but I need to separate the list into two different lists containing the name of the respective element.

Here is the first try at a solution that contains lots of if and instanceof statements:

List<String> itemAList = new ArrayList<>();
List<String> itemBList = new ArrayList<>();

itemList.forEach(item -> {

    if(item instanceof ItemA) {

        itemAList.add(item.getName());
    }        
    else if(item instanceof ItemB) {

        itemBList.add(item.getName());
    }
});

So this works but I gave it some thought on how to avoid the if statements. Since I'm using Java 8 I can do this:

List<String> itemAList = itemList.stream()
    .filter(ItemA.class::isInstance)
    .map(item -> item.getName())
    .collect(Collectors.toList());

List<String> itemBList = itemList.stream()
    .filter(ItemB.class::isInstance)
    .map(item -> item.getName())
    .collect(Collectors.toList());

This works as well but it means I have to process the list two times.

As I said I have no bearing on the implementation of the Item classes so what would be the best way to implement such a behavior?

Greetings

[edit:] Thank you for all the responses. I've learned something new today.

Answer 1

You can group the content based on the class of the elements :

Map<Class, List<String>> grouped = itemList.stream()
        .collect(Collectors.groupingBy(Item::getClass,
                Collectors.mapping(Item::getName, Collectors.toList())));

and access it as:

List<String> itemAList = grouped.get(ItemA.class);

Answer 2

As you said you have different instances you have no control over, so you can't really be sure that only ItemA and ItemB occur in the list.

You're better of using a map, which has the name as the key and a List of Items as values:

Map<String, List<Item>> map = itemList.stream()
    .collect(Collectors.groupingBy(Item::getName, Function.identity()));