Understanding JTable.addColumn()

[Stephan Mueller]

Hello folks,

this is a hacky approach at understanding swing's JTable, based on Sun's Demo Application. After clicking into the table, a new column is added and displayed (my custom datamodel registers as listener to the columnAdded-Event). I'm expecting to have cell-value for this new column = "argh" but the value is the same as it is for the first column (Mary). I don't get this. What am I missing here? The TableDemo.debugColumnAdd output at least assumes that argh is returned for the new column. So why/how are the renderers calling tableModel.getValueAt(...)? Instead of using columnIndex 5 they wrap back to 0.

The main class:

package components;

import java.awt.Dimension;
import java.awt.GridLayout;
import java.awt.event.MouseAdapter;
import java.awt.event.MouseEvent;

import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.JScrollPane;
import javax.swing.JTable;
import javax.swing.table.TableColumn;

public class TableDemo extends JPanel {
    private boolean DEBUG = true;
    private JTable table;

    public TableDemo() {
        super(new GridLayout(1, 0));

        MyDataModel dataModel = new MyDataModel();
        table = new JTable(new MyDataModel());

        table.getColumnModel().addColumnModelListener(dataModel);

        table.setPreferredScrollableViewportSize(new Dimension(500, 70));
        table.setFillsViewportHeight(true);

        if (DEBUG) {
            table.addMouseListener(new MouseAdapter() {
                public void mouseClicked(MouseEvent e) {
                    debugColumnAdd(table);
                }
            });
        }

        // Create the scroll pane and add the table to it.
        JScrollPane scrollPane = new JScrollPane(table);

        // Add the scroll pane to this panel.
        add(scrollPane);
    }

    private void debugColumnAdd(JTable table) {
        System.out.println("Autocreate stuff? "
                + table.getAutoCreateColumnsFromModel());
        TableColumn foo = new TableColumn();
        foo.setHeaderValue("A NEW ONE!");

        table.addColumn(foo);

        int numRows = table.getRowCount();
        int numCols = table.getColumnCount();
        javax.swing.table.TableModel model = table.getModel();

        System.out.println("--------------------------");
        System.out.println("TABLE: Value of data: ");
        for (int i = 0; i < numRows; i++) {
            System.out.print("TABLE:     row " + i + ":");
            for (int j = 0; j < numCols; j++) {
                System.out.print("  " + model.getValueAt(i, j));
            }
            System.out.println();
        }
        System.out.println("--------------------------");
    }

    /**
     * Create the GUI and show it. For thread safety, this method should be
     * invoked from the event-dispatching thread.
     */
    private static void createAndShowGUI() {
        // Create and set up the window.
        JFrame frame = new JFrame("SimpleTableDemo");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        // Create and set up the content pane.
        TableDemo newContentPane = new TableDemo();
        newContentPane.setOpaque(true); // content panes must be opaque
        frame.setContentPane(newContentPane);

        // Display the window.
        frame.pack();
        frame.setVisible(true);
    }

    public static void main(String[] args) {
        // Schedule a job for the event-dispatching thread:
        // creating and showing this application's GUI.
        javax.swing.SwingUtilities.invokeLater(new Runnable() {
            public void run() {
                createAndShowGUI();
            }
        });

    }
}

My custom tableModel:

package components;

import javax.swing.event.ChangeEvent;
import javax.swing.event.ListSelectionEvent;
import javax.swing.event.TableColumnModelEvent;
import javax.swing.event.TableColumnModelListener;
import javax.swing.event.TableModelEvent;
import javax.swing.table.AbstractTableModel;

public class MyDataModel extends AbstractTableModel implements TableColumnModelListener {
    
    int rowCount = 1; 
    static int columnCount = 5;
    
    Object[][] data = {
            { "Mary", "Campione", "Snowboarding", new Integer(5),
                    new Boolean(false) } };
    
    @Override
    public int getRowCount() {
        // TODO Auto-generated method stub
        return rowCount;
    }

    @Override
    public int getColumnCount() {
        // TODO Auto-generated method stub
        return columnCount;
    }

    @Override
    public Object getValueAt(int rowIndex, int columnIndex) {
        System.out.println("columncount = " + columnCount);
        if (columnIndex > 4) {
            System.out.println("argh for GetValueAt " + rowIndex + " " + columnIndex);
            return "argh";
        }
        System.out.println(data[rowIndex][columnIndex] + " for GetValueAt " + rowIndex + " " + columnIndex);
        return data[rowIndex][columnIndex];
    }

    @Override
    public void columnAdded(TableColumnModelEvent e) {
        System.out.println("DATAMODEL found a columnAddedEvent!");
        columnCount++;
        System.out.println("columnCount now " + columnCount);
        this.fireTableChanged(new TableModelEvent(this));	
    }

    @Override
    public void columnRemoved(TableColumnModelEvent e) {
        // TODO Auto-generated method stub
        
    }

    @Override
    public void columnMoved(TableColumnModelEvent e) {
        // TODO Auto-generated method stub
        
    }

    @Override
    public void columnMarginChanged(ChangeEvent e) {
        // TODO Auto-generated method stub
        
    }

    @Override
    public void columnSelectionChanged(ListSelectionEvent e) {
        // TODO Auto-generated method stub
        
    }

}

[Stephan Mueller]

Solution:
TableColumn foo = new TableColumn(); creates a new TableColumn with a model index of 0! That's why the code always returns Mary (getValueAt(0,0)) instead of the expected argh (getValueAt(0,5))
TableColumn foo = new TableColumn(5); is the solution for adding one column, otherwise TableColumn foo = new TableColumn(tableModel.getColumnCount); would be for adding arbitrary numbers of columns.