Collection Casting

Because collections in Apex have a declared type at runtime, Apex allows collection casting.

Collections can be cast in a similar manner that arrays can be cast in Java. For example, a list of CustomerPurchaseOrder objects can be assigned to a list of PurchaseOrder objects if class CustomerPurchaseOrder is a child of class PurchaseOrder.

1public virtual class PurchaseOrder {
2
3    Public class CustomerPurchaseOrder extends PurchaseOrder {
4
5    }
6    {
7        List<PurchaseOrder> POs = new PurchaseOrder[] {};
8        List<CustomerPurchaseOrder> CPOs = new CustomerPurchaseOrder[]{};
9        POs = CPOs;
10    }
11}

Once the CustomerPurchaseOrder list is assigned to the PurchaseOrder list variable, it can be cast back to a list of CustomerPurchaseOrder objects, but only because that instance was originally instantiated as a list of CustomerPurchaseOrder objects. A list of PurchaseOrder objects that is instantiated as such cannot be cast to a list of CustomerPurchaseOrder objects, even if the list of PurchaseOrder objects contains only CustomerPurchaseOrder objects.

If the user of a PurchaseOrder list that only includes CustomerPurchaseOrders objects tries to insert a non-CustomerPurchaseOrder subclass of PurchaseOrder (such as InternalPurchaseOrder), a runtime exception results. This is because Apex collections have a declared type at runtime.