as others have said, static is not polymorphic... it means it is the same for the class, not specific to an instance. i am not sure what you would accomplish by using a static method... but you could have have your extended method implementation return a static value:
public String getExtension() {
return STATIC_CONSTANT;
}
I guess it depends on what you are trying to accomplish with the static method. From your post: "Let me say, everything works OK implemnting a instance getExtension() method in each subclass. But the point is that the method is in fact static in the subclass, has no dependence on the instance. I know I can't do that, I only saying it seems no so strange idea to me."
The problem is you are asking for polymorphic behavior (a change in the behavior of a method for each subclass) from something that defines behavior for the class, not the instance. By definition, static cannot be polymorphic. I hope this helps you with your understanding here of why this is not possible.