Java, change a cell content as a function of another cell in the same row

I’ve revised your sscce to show the alternate approach suggested here. Note the alternate ways to get a Double constant. I’ve also re-factored the String constrants.

Addendum: In helpful comments, @kleopatra observes that querying the model directly will always produce the correct result, but a TableModelListener will only see changes to column 0, not column 1. The simple expedient is to make column 1 non-editable, as its value depends completely on column 0.

@Override
public boolean isCellEditable(int row, int col) {
    return col == 0;
}

The first example below uses DefaultTableModel:

import javax.swing.*;
import javax.swing.table.*;

/** @see https://stackoverflow.com/a/13628183/230513 */
public class ExampleRemoveAddRows extends JFrame {

    public static final String NEGATIVE = "negativ";
    public static final String POSITIVE = "positiv";
    private Object[] columnNames = {"Double", POSITIVE + "https://stackoverflow.com/" + NEGATIVE};
    private Object[][] data = {
        {10d, null},
        {-10.0, null},
        {Double.valueOf(30), null},
        {Double.valueOf("-30"), null}
    };
    private JTable table;
    private DefaultTableModel model;

    public ExampleRemoveAddRows() {
        model = new DefaultTableModel(data, columnNames) {

            @Override
            public Class getColumnClass(int column) {
                return getValueAt(0, column).getClass();
            }

            @Override
            public boolean isCellEditable(int row, int col) {
                return col == 0;
            }

            @Override
            public Object getValueAt(int row, int col) {
                if (col == 1) {
                    double number = (Double) this.getValueAt(row, 0);
                    return (number < 0) ? NEGATIVE : POSITIVE;
                } else {
                    return super.getValueAt(row, col);
                }
            }

            @Override
            public void setValueAt(Object aValue, int row, int col) {
                super.setValueAt(aValue, row, col);
                fireTableCellUpdated(row, 1); // may have changed
            }
        };
        table = new JTable(model);
        table.setPreferredScrollableViewportSize(table.getPreferredSize());
        JScrollPane scrollPane = new JScrollPane(table);
        add(scrollPane);
    }

    public static void main(String[] args) {
        javax.swing.SwingUtilities.invokeLater(new Runnable() {

            @Override
            public void run() {
                ExampleRemoveAddRows frame = new ExampleRemoveAddRows();
                frame.setDefaultCloseOperation(EXIT_ON_CLOSE);
                frame.pack();
                frame.setLocationRelativeTo(null);
                frame.setVisible(true);
            }
        });
    }
}

This variation extends AbstractTableModel:

import java.awt.EventQueue;
import java.util.ArrayList;
import java.util.List;
import javax.swing.*;
import javax.swing.table.*;

/**
* @see https://stackoverflow.com/a/13628183/230513
*/
public class ExampleRemoveAddRows extends JFrame {

    public static final String NEGATIVE = "negativ";
    public static final String POSITIVE = "positiv";

    public ExampleRemoveAddRows() {
        DoubleModel model = new DoubleModel();
        model.add(10.1);
        model.add(-10.2);
        model.add(Double.valueOf(30.1));
        model.add(Double.valueOf("-30.2"));
        JTable table = new JTable(model);
        table.setPreferredScrollableViewportSize(table.getPreferredSize());
        JScrollPane scrollPane = new JScrollPane(table);
        add(scrollPane);
    }

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

            @Override
            public void run() {
                ExampleRemoveAddRows frame = new ExampleRemoveAddRows();
                frame.setDefaultCloseOperation(EXIT_ON_CLOSE);
                frame.pack();
                frame.setLocationRelativeTo(null);
                frame.setVisible(true);
            }
        });
    }

    private class DoubleModel extends AbstractTableModel {

        List<Double> data = new ArrayList<Double>();

        public void add(Double d) {
            data.add(d);
        }

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

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

        @Override
        public String getColumnName(int col) {
            if (col == 0) {
                return "Double";
            } else {
                return POSITIVE + "https://stackoverflow.com/" + NEGATIVE;
            }
        }

        @Override
        public Class<?> getColumnClass(int col) {
            if (col == 0) {
                return Double.class;
            } else {
                return String.class;
            }
        }

        @Override
        public boolean isCellEditable(int row, int col) {
            return col == 0;
        }

        @Override
        public Object getValueAt(int row, int col) {
            if (col == 0) {
                return data.get(row);
            } else {
                double number = (Double) this.getValueAt(row, 0);
                return (number < 0) ? NEGATIVE : POSITIVE;
            }
        }

        @Override
        public void setValueAt(Object aValue, int row, int col) {
            if (col == 0) {
                data.set(row, (Double) aValue);
                fireTableRowsUpdated(row, row);
            }
        }
    }
}