Problem obtaining web service response

I made a web service REST in .NET which is succesfully conected with the app made in android, but the problem is that when I want to obtain the response, String in this case, I can't compare it inside the android app.

The problem is that if I've got r开发者_如何学运维es as my web service response, with a value of "Err" and I do:

if (res == "Err")

It just says they do not match. What could be the problem?

Comparing strings in Java:

if(res.equals("Err")) {
 // equals
}