Why are my JTable rows showing duplicate values when using a custom DataModel?

JTable Duplicate Values in Row

Populating a JTable with a custom DataModel can occasionally lead to the unexpected appearance of duplicate data in the table's rows. While this may suggest an issue with the DataModel, closer inspection often reveals otherwise, leaving the cause of this behavior a mystery.

One possible explanation for this phenomenon lies in the implementation of the JTable itself. When a DataModel is used to populate a JTable, the rendered values in the table are derived directly from the getValueAt method of the DataModel. If this method fails to correctly retrieve the unique value for each cell in the table, the rendered values will be duplicated across multiple rows.

To address this issue, it is essential to ensure that the getValueAt method always returns the intended value for each cell. This can be achieved by properly populating the underlying data structure used by the DataModel. In the case of the provided DataModel, the populate method fills the data ArrayList with rows of data. However, if the code that populates this ArrayList does not correctly assign unique values to each row, the resulting table will display duplicate values.

As a reminder, it is not sufficient to solely modify the getValueAt method to return distinct values, as this will not rectify the underlying issue. Instead, the focus should be on ensuring that the data structure being used to populate the DataModel contains the correct data from the outset. By addressing the data source, it is possible to prevent duplicate values from appearing in the JTable.

To provide a more concrete example, consider the following code snippet:

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

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();
    }
}

In this code, the populate method fills the data ArrayList with rows of data obtained from the Collection object. To avoid duplicate values in the JTable, it is crucial to ensure that each row in the ArrayList contains unique data. This can be achieved by checking for the presence of duplicate data before adding new rows to the ArrayList or by employing a data structure that enforces uniqueness.

By addressing the data source directly, it is possible to prevent duplicate values from appearing in the JTable. This approach is more reliable and efficient than manipulating the getValueAt method, as it ensures the underlying data is accurate from the start.

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