How to run method on private field using reflection?

How to run method on private field using reflection?

I have a resource class as
public class SecureResource {
private HttpServletRequest request;
// some more things here
}
I would like to tun run request.getHeader('X-AUTH') using java reflection?
what I have tried?
Field f = response.getResourceClass().getDeclaredField("request");
f.setAccessible(true);
f.get("X-AUTH");
I get
java.lang.IllegalArgumentException: Can not set
javax.servlet.http.HttpServletRequest field
com.sn.bb.service.SecureResource.request to java.lang.String
What is that I am missing? How can I run request.getHeader('X-AUTH') on f?