JTable and DefaultTableModel

import java.awt.FlowLayout;
import java.util.Random;

import javax.swing.JFrame;
import javax.swing.JTable;
import javax.swing.event.TableModelEvent;
import javax.swing.event.TableModelListener;
import javax.swing.table.DefaultTableModel;

public class Serdar {
    public static void main(String args[]) {
        Random r = new Random();
        JFrame frame = new JFrame("serdar");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.getContentPane().setLayout(new FlowLayout());

        final DefaultTableModel model = new DefaultTableModel();
        JTable table = new JTable(model);
        model.addColumn("ders1");
        model.addColumn("ders2");
        model.addColumn("ders3");
        model.addColumn("ortalama");
        for(int i=0; i<20; i++) {
            Integer[] not = new Integer[4];
            not[0] = r.nextInt(101);
            not[1] = r.nextInt(101);
            not[2] = r.nextInt(101);
            not[3] = (not[0] + not[1] + not[2])/3;
            model.addRow(not);
        }

        model.addTableModelListener(new TableModelListener() {
            public void tableChanged(TableModelEvent e) {
                if(e.getColumn() != 3) {
                    System.out.println(model.getValueAt(e.getFirstRow(), e.getColumn()));
                    int total = 0;
                    for(int i=0; i<3; i++) {
                        total += Integer.parseInt(("" + (model.getValueAt(e.getFirstRow(), i))));
                    }
                    model.setValueAt(total/3, e.getFirstRow(), 3);
                }
            }
        });

        frame.add(table);

        frame.pack();
        frame.setVisible(true);
    }


}