Java Generics Wildcarding With Multiple Classes
I want to have a Class object, but I want to force whatever class it represents to extend class A and implement interface B.
I can do:
Class<? extends ClassA>
Or:
Class<? extends InterfaceB>
but I can't do both. Is there a way to do this?
Actually, you can do what you want. If you want to provide multiple interfaces or a class plus interfaces, you have to have your wildcard look something like this:
<T extends ClassA & InterfaceB>
See the Generics Tutorial at sun.com, specifically the Bounded Type Parameters section, at the bottom of the page. You can actually list more than one interface if you wish, using & InterfaceName for each one that you need.
This can get arbitrarily complicated. To demonstrate, see the JavaDoc declaration of Collections#max , which (wrapped onto two lines) is:
public static <T extends Object & Comparable<? super T>> T
max(Collection<? extends T> coll)
why so complicated? As said in the Java Generics FAQ: To preserve binary compatibility.
It looks like this doesn't work for variable declaration, but it does work when putting a generic boundary on a class. Thus, to do what you want, you may have to jump through a few hoops. But you can do it. You can do something like this, putting a generic boundary on your class and then:
class classB { }
interface interfaceC { }
public class MyClass<T extends classB & interfaceC> {
Class<T> variable;
}
to get variable that has the restriction that you want. For more information and examples, check out page 3 of Generics in Java 5.0. Note, in <T extends B & C> , the class name must come first, and interfaces follow. And of course you can only list a single class.
You can't do it with "anonymous" type parameters (ie, wildcards that use ? ), but you can do it with "named" type parameters. Simply declare the type parameter at method or class level.
import java.util.List;
interface A{}
interface B{}
public class Test<E extends B & A, T extends List<E>> {
T t;
}
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