@Override
    public String toString() {
        StringBuilder sb = new StringBuilder();

        String space = "";
        for (FieldOrder spec : fieldOrders)   {
            sb.append(space);
            if (spec.getSortOrder() == Order.DESCENDING) {
                sb.append("-");
            } else {
                sb.append("+");
            }
            sb.append(spec.getFieldName());
            space = " ";
        }
        return sb.toString();
    }
Creates a new sorting from the given string and returns it, or returns null if the argument does not contain
any sorting criteria (e.g it is null or the empty string)
container-search/src/main/java/com/yahoo/search/query/Sorting.java