Java Programming

Hi,

I have a java based-simulation that contains the following code :

EDProtocol  prot = null;
boolean check=false;
			try {
				*prot = (EDProtocol) ev.node.getProtocol(pid);*
				prot.processEvent(ev.node, pid, ev.event);
                                *check= (ev.node.getProtocol(pid) instanceof DRingProtocol);*
			} catch (ClassCastException e) {
				throw new IllegalArgumentException("class: "+ ev.node.getProtocol(pid).getClass()+ "; is instance of DRingProtocol: "+ check);
			}

However, sometimes I get this weird error:
"Exception in thread "main" java.lang.IllegalArgumentException: Class: DRingProtocol; is instance of DRingProtocol; false"

How can "prot" not be an instance of its class???

Thanks

Mannous