Java Generics With a Class an Interface - Together

Question

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 lt   extends ClassA gt    Or   Class lt   extends InterfaceB gt    but I can t do both   Is there a way to do this

User · Answer

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 lt E extends B  amp  A  T extends List lt E gt  gt        T t

User · Answer

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    lt T extends ClassA  amp  InterfaceB gt    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  amp  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  lt T extends Object  amp  Comparable lt   super T gt  gt  T                                            max Collection lt   extends T gt  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 lt T extends classB  amp  interfaceC gt        Class lt T gt  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  lt T extends B  amp  C gt   the class name must come first  and interfaces follow   And of course you can only list a single class

User · Answer

Here s how you would do it in Kotlin fun  lt T gt  myMethod item  T  where T   ClassA  T   InterfaceB         your code here

[java] Java Generics With a Class & an Interface - Together

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