tell me the format of the contents of a jtable?

Your thinking is correct. Within the custom TableCellRenderer, you can actually check which column/row/cell is rendered and subsequently assign a column/row/cell specific formatting.

 public static class CustomTableCellRenderer extends DefaultTableCellRenderer {    
        @Override
        public Component getTableCellRendererComponent(JTable table, Object value, boolean isSelected, boolean hasFocus, int row, int column) {
            DefaultTableCellRenderer c = (DefaultTableCellRenderer) super.getTableCellRendererComponent(table, value, isSelected, hasFocus, row, column);

            // center everything in the first column
            if (column == 0) {
                c.setHorizontalAlignment(JLabel.CENTER);
            }

            // the background and border of the first cell should be gray
            if (column == 0 && row == 0) {
                c.setBackground(Color.GRAY);
                c.setBorder(BorderFactory.createMatteBorder(0, 5, 0, 5, Color.GRAY));
            }

            return c;
        }
 }

Please note that the DefaultTableCellRenderer is called for each individual cell.

All available formatting functions are well described in the respective documentation: https://docs.oracle.com/javase/10/docs/api/javax/swing/table/DefaultTableCellRenderer.html