you can not check a primitive (double, float, long, int, char, short, byte, boolean) datatype for null.
primitives have a value always. you can only check references to Objects for null. there is a wrapper Object for each of the primitives (Double, Float,, Long, Integer, ...) if you use these, then you can check for null.
if you use java 1.5/5.0 then you should read about "autoboxing/unboxing", a feature which allows you to write code like this:
here primitives and wrapper objects are mixed. of course there are a some rules when/how autoboxing happens (e.g. what if you assign null to a primitive and so on). and it's important you understand these...