private void init() {
    setDataSource();
    addGeneratedColumn("title", new TitleColumnGenerator());
    addGeneratedColumn("description", new DescriptionGenerator());
    setColumnExpandRatios();
    setItemDescriptionGenerator(new TooltipGenerator());
}

protected class TooltipGenerator implements ItemDescriptionGenerator{
        private static final long serialVersionUID = 1L;

        @Override
        public String generateDescription(Component source, Object itemId, Object propertyId) {
            TaskRow taskRow = (TaskRow)itemId;
            if("description".equals(propertyId)){
                return taskRow.getDescription();
            }else if("title".equals(propertyId)){
                return taskRow.getTitle();
            }else if("category".equals(propertyId)){
                return taskRow.getCategory().toString();
            }else if("operation".equals(propertyId)){
                return taskRow.getOperation().toString();
            }else if("resourcePointer".equals(propertyId)){
                return taskRow.getResourcePointer();
            }else if("taskState".equals(propertyId)){
                return taskRow.getTaskState().toString();
            }
            return null;
        }
    }

Node node = ...;
Item item = tree.addItem(node);

public String generateDescription(Component source, Object itemId, Object propertyId) {
    if (itemId instanceof Node) {
        Node node = (Node) itemId;
        ...