public TableCell build() {
            final String cssClass = NullSafe.stream(cssClasses)
                    .collect(Collectors.joining(" "));
            if (value == null) {
                return new TableCell(SafeHtmlUtils.EMPTY_SAFE_HTML, header, colspan, cssClass);
            }
            return new TableCell(value, header, colspan, cssClass);
        }
Add single class or space separated classes to the cell
stroom-core-client-widget/src/main/java/stroom/widget/util/client/TableCell.java