Type safe linkage is ensured by establishing a constraint if a class references an attribute that is contained in another class. This constraint acts as a "promise" to later ensure type safe linkage. At some point later--such as at the earliest time that the type is loaded by both loaders--the constraint is verified. This may be accomplished by verifying that the type for the attribute is the same regardless of whether it is loaded by a loader that defines the referencing class or a loader that defines the referred class. If the constraint is not met, an error message is provided.