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 with sharing class PurchaseOrder {
2}

1public with sharing class CustomerPurchaseOrder extends PurchaseOrder {
2}

1List<CustomerPurchaseOrder> CPOs = new List<CustomerPurchaseOrder>();
2List<PurchaseOrder> POs = CPOs;
3
4// This cast succeeds because POs still has the runtime type List<CustomerPurchaseOrder>.
5List<CustomerPurchaseOrder> restored = (List<CustomerPurchaseOrder>) POs;
6
7// This cast fails at runtime because genericOrders was instantiated as List<PurchaseOrder>.
8// List<PurchaseOrder> genericOrders = new List<PurchaseOrder>();
9// List<CustomerPurchaseOrder> invalid = (List<CustomerPurchaseOrder>) genericOrders;

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 CustomerPurchaseOrder 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.