Can anybody tell me what type of applications java can never be used for or shud not be used for?

Two reasons I can think of off the top of my head are 1) that Java (or rather, the current crop of JVMs) is not a real-time system, mostly due to the delays that can happen due to GC, and 2) that Java has not seen the kind of QA you'd want in a system on which lives depend. Anything that's highly platform dependant. Java is inherently multi-platform and you'll have great pains to tie your code to a specific platform.

For nuclear power plants, aircraft controls, etc, people tend to use programming which has a very small "footprint, ie fewer kB in the whole code than there are MB in Java, because (as Ulf Dittmer says), intensive testing is much easier on a small language.