Unfortunately, there is no setToolTipText() method for cells in a JTable component. For a cell to show a tool tip, the renderer for that cell must set the tool tip text on the returned component. See e928 Creating a Custom Cell Renderer in a JTable Component for an example of a renderer that sets a tool tip. 
If you cannot modify the renderer, you can override the table's prepareRenderer() method and explicitly set the tool tip on the returned component. 

    // This table displays a tool tip text based on the string
    // representation of the cell value
    JTable table = new JTable() {
        public Component prepareRenderer(TableCellRenderer renderer,
                                         int rowIndex, int vColIndex) {
            Component c = super.prepareRenderer(renderer, rowIndex, vColIndex);
            if (c instanceof JComponent) {
                JComponent jc = (JComponent)c;
                jc.setToolTipText((String)getValueAt(rowIndex, vColIndex));
            }
            return c;
        }
    };