I'm guessing; a lot.

That is correct.

I'm *very* suspicious, which is why I tried to change Double.class to
Integer.class, and String.class (my table contains all three types).
None would work. Only Object.class works. I'm assuming that there's
something very basic that I'm missing. (I've just started programming
Java, so I know .. well .. almost nothing about it).

I don't use setValueAt(). Do I need to? Could that be the problem?

FWIW, I included the code (I already know that the cell-code is
suboptimal, but it's just test code):

--------------------------------------------
class TransListCellRenderer extends DefaultTableCellRenderer
{
    private Color c1 = new Color(255, 255, 255);
    private Color c2 = new Color(250, 230, 250);
    public Component getTableCellRendererComponent
        (JTable table, Object value, boolean isSelected,
        boolean hasFocus, int row, int column)
    {
        Component cell = super.getTableCellRendererComponent
            (table, value, isSelected, hasFocus, row, column);
        System.out.println("Check!");
        if(!isSelected)
        {
            if((row % 2) == 0)
                cell.setBackground(c1);
            else
                cell.setBackground(c2);
        }

        cell.setForeground(Color.black);
        if(value instanceof Double )
        {
            Double amount = (Double)value;
            if(amount.doubleValue() < 0.0)
            {
                cell.setForeground(Color.red);
            }
        }

        // test code: illustrates the problem.
        cell.setForeground(Color.red);

        return cell;
    }
}

class TransListNode
{
   private int id;
   private String category;
   private java.util.Date date;
   private double amount;
   private String description;

    public TransListNode(Connection con, int id) throws java.sql.SQLException {
        String Q = "SELECT T.ID,C.TITLE,T.DATE,T.AMOUNT,T.DESCRIPTION FROM " +
"TRANSACTIONS AS T, CATEGORIES AS C WHERE T.CATEGORY_ID=C.ID AND " +
"T.ID=" + id;

        System.out.println(Q);
        Statement stmt = con.createStatement();
        ResultSet rs = stmt.executeQuery(Q);
        System.out.println(rs);

        if(rs.next())
        {
            id = rs.getInt(1);
            category = rs.getString(2);
            date = rs.getDate(3);
            amount = rs.getDouble(4);

            Clob clob = rs.getClob(5);
            if(rs.wasNull())
                description = "<null>";
            else
            {
                int len = (int)clob.length();
                if(len > 64)
                    description = clob.getSubString(1, 64) + "...";
                else
                    description = clob.getSubString(1, len);
            }
        }

        // Clean up..
        rs.close();
        stmt.close();
    }

    public int getId() {
        return id;
    }
    public String getCategory() {
        return category;
    }
    public java.util.Date getDate() {
        return date;
    }
    public String getDescription() {
        return description;
    }
    public double getAmount() {
        return amount;
    }

    public String toString() {
        return "" + id + "," + category + "," + date + "," + amount + "," +
            description;
    }
}

class TransListTableModel extends AbstractTableModel
{
    private String[] columnNames = { "Id", "Category", "Date", "Amount",
        "Description" };
    private Vector<TransListNode> rowList;

    public TransListTableModel() {
        rowList = new Vector<TransListNode>();
    }

    public void addRow(TransListNode tln) {

        // Add to database!!
        try {
            rowList.add(tln);
        }
        catch(Exception e) {
            JdbcException jdbcExc = new JdbcException(e);
            jdbcExc.handle();
        }
    }

    public int getColumnCount() {
        return columnNames.length;
    }
    public int getRowCount() {
        return rowList.size();
    }
    public String getColumnName(int col) {
        return columnNames[col];
    }
    public Object getValueAt(int row, int col) {
        TransListNode tln = (TransListNode)rowList.get(row);
        switch(col)
        {
            case 0:
                return tln.getId();
            case 1:
                return tln.getCategory();
            case 2:
                return tln.getDate();
            case 3:
                return tln.getAmount();
            case 4:
                return tln.getDescription();
        }
        return null;
    }
    public Class getColumClass(int col) {
        System.out.println("Returning: " + getValueAt(0, col).getClass());
        return getValueAt(0, col).getClass();
    }
}
--------------------------------------------

I'm assuming that since this doesn't work, I'm using getColumnClass()
and getValueAt() all wrong.

..or I'm simply missing something else.

As I said, I'm a Java newbie, please keep it simple if possible.

PS. For future reference; how much code is accepted per post here
without people throwing a fit?

--
Kind Regards,
Jan Danielsson

Wadd'ya know! :-)  Thanks!

Makes me wonder: I wonder if those other having the same problem as I
had copied code from the same webpage as I did?

How come? When does getColumnClass() get called, other than when there
are rows?

This limitation I am aware of, but since the colums are directly mapped
to columns in an SQL database, that doesn't matter.

Thanks for spotting that typo..