Add column to JTable that has some rows before adding

Question

I have a JTable that contains 4 columns and for example 10 rows. I want to add a new column and put the maximum value for each row in that.

Here is the function that I use to compute the maximum and put them into the new column:

private void jFindmaximumActionPerformed(java.awt.event.ActionEvent evt) {                                             
        DefaultTableModel model = (DefaultTableModel)jMYtable.getModel();
        TableColumn col = new TableColumn(model.getColumnCount());
        String headerLabel = "Maximum";
        col.setHeaderValue(headerLabel);
        jMYtable.addColumn(col);

        model.addColumn(headerLabel.toString(), new Object [0]);  
        double[] MAX = new double[ndata[0].length];
        for(int i = 0; i < ndata[0].length; i++){
            MAX[i] = ndata[i][0];
            for(int j = 0 ; j < ndata.length; j ++){
                if(ndata[j][i] > MAX[i]){
                    MAX[i] = ndata[j][i];
                }
            }
            model.setValueAt(MAX[i],i,4);
        }
        jMYtable.revalidate();
 } 

If my table does not have any row when I call this method the new column is added and shows up on the interface. However, when I populate my table with 10 rows and call this action, table is not acting properly:

1. There is a gui redraw problem where the new column will show only after mouse goes on the jtable; and
2. The new column does not have any data.

Please help.

Answer 1

I tested your code and while I didn't get the exact same error, there's a couple of weird things:

First, you are calling both jMyTable.addColumn(...) and model.addColumn(...). Which is a mistake. You only need to call the model's addColumn. Adding also a column with the JTable method will create inconsistencies, because the TableModel and the TableColumnModel are not synchronized.

Your code to add a new column into the model, and the calculation of the maximums is wrong. i should move through rows, and j should move through columns, and then check if we should do the MAX[i] = ndata[i][j]; assignment. And while passing new Object[0] doesn't appear to cause any issues (because that constructor with a null value is valid) it doesn't make any sense.

It should be something like this:

private void jFindmaximumActionPerformed(java.awt.event.ActionEvent evt) {  
    DefaultTableModel model = (DefaultTableModel)jMYtable.getModel();
    String headerLabel = "Maximum";

    Object[] MAX = new Object[ndata.length];

    for(int i = 0; i < ndata.length; i++){
         MAX[i] = ndata[i][0];
          for(int j = 0 ; j < ndata[0].length; j ++){
                  if(ndata[i][j] > MAX[i]){
                        MAX[i] = ndata[i][j];
                  }
             }
    }
    model.addColumn(headerLabel, MAX);  
}

On a side note, if you ever were to implement your own TableModel, extending AbstractTableModel, when the memory structure (created and handled by yourself) that contains the table data changes, you have to update the table with the new values or new column/rows changes using an assortment of methods in the AbstractTableModel class to notify the JTable: fireTableDataChanged(), fireRowsInserted(), fireTableStructureChanged()...

UPDATE:

Here's a simple test code that adds columns to a JTable.

import java.awt.BorderLayout;
import java.awt.GridLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.JScrollPane;
import javax.swing.JTable;
import javax.swing.SwingUtilities;
import javax.swing.table.DefaultTableModel;

public class SimpleColumnAdderFrame
{
    private DefaultTableModel _model;

    public static void main(String[] args) 
    {
        SwingUtilities.invokeLater(new Runnable() {
            public void run() {
                new SimpleColumnAdderFrame().initUI();
            }
        });
    }

    public void initUI()
    {
        JFrame frame = new JFrame();        
        Object[][] data = {
                {1,2,3},
                {10,20,30}
        };
        String[] columnNames = {"Column 1", "Column 2", "Column 3"};        
        _model = new DefaultTableModel(data, columnNames);
        JTable table = new JTable(_model);

        JButton addColumnButton = new JButton("Add column");
        addColumnButton.addActionListener(new ActionListener(){
            public void actionPerformed(ActionEvent e)
            {
                int columns = _model.getColumnCount();
                int rows = _model.getRowCount();
                Object[] columnData = new Object[rows];
                for (int i = 0; i < rows; ++i)
                {
                    columnData[i] = (int) ((columns + 1)*(Math.pow(10, i))); // <- Unnecessarily complicated cell value 
                }
                _model.addColumn("New Column " + (columns + 1), columnData);
            }   
        });

        JButton addRowButton = new JButton("Add row");
        addRowButton.addActionListener(new ActionListener(){
            public void actionPerformed(ActionEvent e)
            {
                int columns = _model.getColumnCount();
                int rows = _model.getRowCount();
                Object[] rowData = new Object[columns];
                for (int i = 0; i < columns; ++i)
                {
                    rowData[i] = (int) ((i+1)*(Math.pow(10, rows))); // <- Unnecessarily complicated cell value 
                }
                _model.addRow(rowData);
            }
        });

        JPanel buttonPanel = new JPanel(new GridLayout(1,2,2,2));
        buttonPanel.add(addColumnButton);
        buttonPanel.add(addRowButton);

        frame.add(new JScrollPane(table), BorderLayout.CENTER);
        frame.add(buttonPanel, BorderLayout.PAGE_END);
        frame.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);
        frame.pack();
        frame.setVisible(true);
    }

}