Java compilation error when returning Generic Class Type

Question

public interface Parent{ } public class Child implements Parent{ } public <T extends Parent> Class<T> getClass(){ return Child.class; // compile error, add cast to Class<T> } 

I expect errors above the code, but I get a compilation error when I return Child.class.

Answer 1

You cannot tell Java to always return Child.class regardless of T There may be other classes extending from the parent that are not Child . Also, since you are not using T anywhere, your code does not make much sense. Two possible solutions:

 public Class<? extends Parent> getAClass() { return Child.class; } 

or maybe

 public <T extends Parent> Class<T> getAClass(Class<T> clazz){ return clazz; } 

The second example compiles, but it probably makes sense if T was declared at the class level.

In addition, I renamed your method to avoid a name clash with the existing getClass() method.

Answer 2

Check out the following

 public class Main { public interface Parent{} public class Child implements Parent{} public class BadChild implements Parent {} public static <T extends Parent> Class<T> getClazz() { return Child.class; // compile error, add cast to Class<T> } public static void main(String[] args) { Class<BadChild> clazz = Main.<BadChild>getClazz(); } } 

The call method getClazz() with type BadChild valid for the caller side. If you can compile your method that will lead to runtime errors. That is why it is forbidden.

Answer 3

If this code was legal, you could write:

 Class<SomeOtherChildClass> clazz = instance.getAClass(); 

(assuming SomeOtherChildClass also extending Parent ), which would obviously fail at runtime for an existing implementation.

I am not suggesting a “fixed” code because I am not sure what you are trying to do.