Personalizar JTable: Problema resuelto

import javax.swing.JOptionPane;
import javax.swing.event.TableModelEvent;
import javax.swing.event.TableModelListener;
import javax.swing.table.TableModel;
/**
 * @web https://www.jc-mouse.net
 * @author Mouse
 */
public class TModelListener implements TableModelListener  {

    /**
 * @param e TableModelEvent
 */
    public void ajuste( TableModelEvent e ){

        //solo si se edita la tabla
        if ( e.getType() == TableModelEvent.UPDATE ) {

            //se obtiene el modelo
            TableModel modelo = ( (TableModel) ( e.getSource() ) );            
            int fila = e.getFirstRow();
            int columna = e.getColumn();
            if ( columna == 3 ) {return;}

             try {                
                 //obtiene valor de celda editable
                int a = ((Number) modelo.getValueAt(fila,columna)).intValue();
                //obtiene valor de celda no editable
                int b = ((Number) modelo.getValueAt(fila,columna+1)).intValue();

                //restricción
                if( a <= 0){
                   JOptionPane.showMessageDialog( null , "Error: El valor debe ser enterio positivo.");
                   modelo.setValueAt( null , fila, columna); 
                   modelo.setValueAt( null , fila, columna+2); 
                   return;
                }

                modelo.setValueAt( a + b , fila, columna+2);

            } catch (NullPointerException ex) { /*...*/ }             
        }
    }

    @Override
    public void tableChanged(TableModelEvent e) {
        ajuste( e );
    }

}

import javax.swing.table.DefaultTableModel;
/**
 * @web http://jc-mouse.net/
 * @author Mouse
 */
public class TModel extends DefaultTableModel{
    Class[] types = new Class [] {
                java.lang.Object.class, java.lang.Integer.class, java.lang.Integer.class, java.lang.Integer.class
            };
    boolean[] canEdit = new boolean [] {
                false, true, false, false
            };

    public TModel(){
        //unos valores por default 🙂
         super( new Object[][] { { "Producto A", 23, 21, (23+21) }, 
                                 { "Producto B", 112, 2, (112+2) },
                                 { "Producto C", 22, 1, (22+1) },
                                 { "Producto D", 544, 2, (544+2) },
                                 { "Producto E", 12, 3, (12+3) }
                               },
         new String[] { "Producto", "Valor editable", "Valor Fijo","Total" });         

    }

    @Override
    public Class getColumnClass(int columnIndex) {
                return types [columnIndex];
            }

    @Override
    public boolean isCellEditable(int rowIndex, int columnIndex) {
          return canEdit [columnIndex];
    }
}

import java.awt.Color;
import java.awt.Component;
import java.awt.Font;
import javax.swing.JTable;
import javax.swing.table.DefaultTableCellRenderer;
/**
 * @web http://jc-mouse.net/
 * @author Mouse
 */
public class CellRenderer extends DefaultTableCellRenderer{
    private Font bold = new Font( "Arial",Font.BOLD ,12 );

    @Override
    public Component getTableCellRendererComponent ( JTable table, Object value, boolean selected, boolean focused, int row, int column )
    {

        if( value == null )
        {
            this.setBackground( new Color(255,0,0) );
            this.setText( null );

        }else
        {
            this.setText( "" + Integer.valueOf( value.toString() ) );    
            this.setBackground( new Color(0,255,0) );
        }

        this.setForeground( (selected)? new Color(0,0,255):new Color(0,110,10) );            
        this.setFont(bold);            
        return this;
    }
}