Установите значение в методе getTableCellEditorComponent(..)
.
Пример:
public static void main(String... args) {
JFrame frame = new JFrame("Test");
JTable table = new JTable(10, 2);
JComboBox box = new JComboBox(new String[] {"A", "B", "C"});
table.setDefaultEditor(Object.class, new DefaultCellEditor(box) {
@Override
public Component getTableCellEditorComponent(JTable table,
Object value, boolean isSelected, int row, int column) {
return super.getTableCellEditorComponent(
table,
table.getValueAt(Math.max(row-1, 0), column),
isSelected,
row,
column);
}
});
frame.add(table);
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.setSize(400, 300);
frame.setVisible(true);
}