Somewhere in your code:
var ac : ArrayCollection = new ArrayCollection();
var newItem : Items = new Items();
newItem.name = 'some value';
newItem.count = 1;
ac.addItem(newItem);
newItem = new Items();
newItem.name = 'some other value';
newItem.count = 2;
ac.addItem(newItem);
newItem = new Items();
newItem.name = 'Yet another value';
newItem.count = 3;
ac.addItem(newItem);
...
It may be worth nothing that ArrayCollections are not typed in the same way that a Vector may be, so there is nothing you can do to force all items in your collection to be of the Items type. [Unless you extend ArrayCollection somehow and override the addItem / addItemAt methods to throw errors if the type is wrong.