I don't know Terrence Parr or his recommendations, but if it were me I would probably write something like

String opKeyStr = request.getParameter("opkey");
if ((null == opKeyStr) || ("".equals(opKeyStr.trim))) {
// do some error handling, return INVALID_LONG
}
long opKey = INVALID_LONG;
try {
    opKey = Long.parseLong(opKeyStr);
} catch (NumberFormatException e) {
// do some error handling, return INVALID_LONG
}

return opKey;