gistfile1.java

        table.addCellPreviewHandler(new CellPreviewEvent.Handler<Document>() {
            @Override
            public void onCellPreview(CellPreviewEvent<Document> event) {
                if (BrowserEvents.MOUSEOVER.equals(event.getNativeEvent().getType())) {
                    overDocument = event.getValue();
                }
            }
        });

        table.addAttachHandler(new Handler() {
            @Override
            public void onAttachOrDetach(AttachEvent event) {
                if (event.isAttached()) {
                    initTooltip(table);
                }
            }
        });
    }       
     
    private void initTooltip(CellTable<Document> documents) {       
        TooltipOptions options = new TooltipOptions();

        options.withContent(new TooltipContentProvider() {
            @Override
            public String getContent(Element element) {
                return documentTemplates.documentThumbnailTooltip(overDocument.getThumbnailUrl()).asString();
            }
        });

        options.withPlacement(TooltipPlacement.LEFT);
        options.withSelector(fontsResources.icons().icon_preview());
        options.withContainer("element");

        $(documents).as(Tooltip.class).tooltip(options);