Implementation

 

The following issues arise when implementing this pattern.

• Arguments to specificRequest. Projecting polymorphic behavior will usually require adaptation of the signatures of the various specificRequests. This can be complicated when, for instance, some require arguments, whereas others do not. The implementer must decide whether to expose those arguments in the polymorphic interface or to insulate the client from them.
• Where does the code go? As mentioned in Section 6, the external class hierarchy must be maintained in parallel with the original classes. The implementer must carefully select the source files in which co-dependent code, such as signature adaptation, is implemented.
• External Polymorphism is not an Adapter. The intent of the Adapter pattern is to convert an interface to something usable by a client. External Polymorphism, on the other hand, focuses on providing a new base for existing interfaces. A ``serendipitous'' use of External Polymorphism would find all signatures for specificRequest identical across all disparate classes, and thus not require the use of SignatureAdapter; it is in this situation where it is most apparent that External Polymorphism is not an Adapter.