Hello everyone,
After I created a JTable with:
JTable MaTable =new JTable();
MaTable.setModel(new DefaultTableModel(new Object [N][1], new String [1]));

I wanted to add new columns to it, so I did this:
TableColumn Colonne = new TableColumn();
MaTable.addColumn(Colonne);

The operation did worked, even the new columns displayed correctly, but, when I set a value for one of the new cells, all the others (newly created) cells are modified!
It's like all new cells have the same index.
Did I miss something in my code that should correct the index? (if the problem is with the index).
I'm waiting for your precious help.

post the code you use for editing the single new cell.

I just used mytable.setValueAt(value,i,j) to edit new cells
nothing special

Here's the solution if you're interested.
Like I suspected, it was an Index problem. In my code listed above, I did not tell TableColumn which column in the model it refers to, so it defaults to 0 (or something common for all new cells). I should-ed type TableColumn colonne = new TableColumn(diagramme.getColumnCount());
So TableColumn refer every new column at the end of the table (that makes sens).
So, that was the major problem, but I did modified the code in order to work properly:
diagramme.setAutoCreateColumnsFromModel(false);
DefaultTableModel model = DefaultTableModel)diagramme.getModel();
TableColumn colonne = new TableColumn(diagramme.getColumnCount());
diagramme.addColumn(colonne);
model.addColumn(colonne);