if (vn == null)
if (v == null)
    public void touch(VTDNav v) {        if (vn == null)            throw new IllegalArgumentException(" VTDNav instance can't be null");        depth = vn.context[0];        index = (depth != 0) ? vn.context[0] : vn.rootIndex;        vn = v;        prevLocation = -1;        lcIndex = -1;        lcUpper = -1;        lcLower = -1;    }