public Field addField(String fieldName) throws IllegalArgumentException {
checkNotNull(fieldName, "fieldName must not be null");
checkArgument(fieldName.length() > 0, "fieldName must not be empty");
public Field addField(String fieldName) throws IllegalArgumentException {
checkNotNull(fieldName, "fieldName must not be null");
checkArgument(fieldName.length() > 0, "fieldName must not be empty");