Question

I want to have a collection of subclass objects but with my generic type implementation as it is now gives an error at allItems.add(item); because allItems does not hold Item types. So how can I change the below code not give an error?

public class ItemManager {
    public static Collection<? extends Item> allItems;
    ...
    public static boolean addItem(Item item){
        return allItems.add(item);
    }
}

A new item might be added as:

itemManager.add(new Bomb());

Is there a way to change addItem to:

public static boolean addItem([all subclasses of Item] item) { ... }

or maybe change allItems so it can accept receiving an Item and a subclass of Item?

Answer 1

The collection should be declared as Collection<Item>.

Collection<? extends Item> means: a collection of some unknown class which is or extends Item. You can’t add anything to such a collection, since you don’t know the type of objects it holds.