public String getUniqueID() {
        if (StringUtils.isNotEmpty(guid)) {
            return guid;
        }
        if (StringUtils.isNotEmpty(geocode)) {
            return geocode;
        }
        throw new IllegalStateException("Trackable must have at least one of geocode or guid");
    }
Check whether this trackable has a corresponding URL.
main/src/main/java/cgeo/geocaching/models/Trackable.java