If the key is entered as null, the instance is reused according to the order of each item. For example, if E is added last in A, B, C, D, the previous instance is reused because the order of A, B, C, D is the same. However, if an item is added in the center, the order of C, D is changed so a new instance is allocated.

If a key is provided, the instance is reused even if the order is changed because the key itself caches the item.