The Artima Developer Community
Sponsored Link

Interface Design by Bill Venners
Prefer polymorphism over instanceof and downcasting


Interface Design | Contents | Previous | Next

Use getPlace() as an example of instanceof. Use perhaps values() and keySet() of LinkMap as example of polymorphism and dynamic binding.

Guideline could be simplified to prefer polymorphism to instanceof.

Try to design things so clients don't have to use instanceof and downcasting. This is where your modeling of abstract types comes in. When you design class Animal, you say what it means to be an animal by saying what services every Animal provides (the syntax and semantics). A good example of this is Set and List and Collection. These rich types allow abstract return types that clients don't usually have to downcast. In your APIs, set things up so people don't have to use instanceof. Sometimes, though, you may find yourself requiring clients to downcast. Show LinkMap, ModifiableLinkMap, DynamicLinkMap. Know a LinkMap is coming, but don't know whether its a DynamicLinkMap unless you use instanceof. If so, can downcast to DynamicLinkMap.

Downcasting and instanceof

The OO Way

Why Design with Types?

Mention UI factory object in Service UI API, in that "UIFactory" is not a type that all UI factory interfaces implement. It is a concept. Client must use the equivalent of instanceof and downcasting. Here, to separate the codebases, there's a different way to find out the UI factory types. (UIFactoryTypes attribute). But the reason we did it that way was we didn't feel we could predict what was coming. By leaving it open, by forcing clients to look for interfaces they understand, we made the UI factory concept more amenable to future changes. Same thing with the service context object in the current Place API. But in general, should avoid the instanceof and downcasting thing and go for specific types (i.e, UIFactory rather than concepts, (i.e., UI factory).

The real trouble with reflection philosophically, which I want to emphasize here, is that you lose semantics when you just look at message signatures. What types really add is the semantics to go along with the message signatures. Using reflection in Java is like using a weakly typed language. If there's a method with an appropriate signature, invoke it. What I said in most recent seminar, "How can I ask you to do something for me, if I don't know what you are going to do when I ask?"

Sponsored Links

Copyright © 1996-2019 Artima, Inc. All Rights Reserved. - Privacy Policy - Terms of Use