Question

I have a JTable populated with a custom DataModel (pasted below) and when I call the populate() method, it appears to populate the table with duplicate data – each row is filled with the same value over and over again. However, on closer inspection (by simply println()ing the ‘data’ field), the data model isn’t at fault – it holds correct data, in the format I expect. What gives?

import java.util.ArrayList;    
import javax.swing.table.AbstractTableModel;

@SuppressWarnings("serial") // we don't expect this app to ever use serialized classes.  EVER.
public class CollectionDataModel extends AbstractTableModel {
    private ArrayList<ArrayList<String>> data;

    public CollectionDataModel() {
        data = new ArrayList<ArrayList<String>>();
    }

    @Override
    public int getColumnCount() {
        if(data.isEmpty()) return 0;
        return data.get(0).size();
    }

    @Override
    public int getRowCount() {
        return data.size();
    }

    @Override
    public Object getValueAt(int rowIndex, int columnIndex) {
        if(rowIndex > getRowCount()) return null;
        if(columnIndex > getColumnCount()) return null;
        return data.get(rowIndex).get(columnIndex);
    }

    public void populate(Collection c) {
        data.clear();
        for(Item i : c.getItems()) {
            ArrayList<String> row = new ArrayList<String>();
            for(Property p : i.getProperties().values()) {
                row.add(p.toString());
            }
            data.add(row);
        }
        fireTableDataChanged();
    }

}

Answer 1

Here’s a complete example that may prove helpful. As the sample Map is unmodifiable, I refer you to @mKorbel’s example on how to override isCellEditable() and setValueAt().

import java.awt.EventQueue;
import java.awt.GridLayout;
import java.util.Map;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.JScrollPane;
import javax.swing.JTable;
import javax.swing.table.AbstractTableModel;

/** @see https://stackoverflow.com/questions/9132987 */
public class EnvTableTest extends JPanel {

    public EnvTableTest() {
        this.setLayout(new GridLayout());
        this.add(new JScrollPane(new JTable(new EnvDataModel())));
    }

    private static class EnvDataModel extends AbstractTableModel {

        private Map<String, String> data = System.getenv();
        private String[] keys;

        public EnvDataModel() {
            keys = data.keySet().toArray(new String[data.size()]);
        }

        @Override
        public String getColumnName(int col) {
            if (col == 0) {
                return "Key";
            } else {
                return "Value";
            }
        }

        @Override
        public int getColumnCount() {
            return 2;
        }

        @Override
        public int getRowCount() {
            return data.size();
        }

        @Override
        public Object getValueAt(int row, int col) {
            if (col == 0) {
                return keys[row];
            } else {
                return data.get(keys[row]);
            }
        }
    }

    private void display() {
        JFrame f = new JFrame("EnvTableTest");
        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        f.add(this);
        f.pack();
        f.setLocationRelativeTo(null);
        f.setVisible(true);
    }

    public static void main(String[] args) {
        EventQueue.invokeLater(new Runnable() {

            @Override
            public void run() {
                new EnvTableTest().display();
            }
        });
    }
}