public void read(FieldBase field, ByteFieldValue value) {
        try {
            String dataParsed = reader.getElementText();
            try {
                value.assign(Byte.valueOf(dataParsed));
            } catch (Exception e) {
                throw newDeserializeException(field, "Invalid byte \"" + dataParsed + "\".");
            }
        } catch (XMLStreamException e) {
            throw newException(field, e);
        }
    }
Optional test and set condition. Common for document/update/remove elements
This variable is either set in VespaXMLFieldReader#read (reader for document)
or in VespaXMLUpdateReader#read (reader for update).
document/src/main/java/com/yahoo/vespaxmlparser/VespaXMLFieldReader.java