私はあなたの目標をより明確に理解したので、JOptionPaneをエミュレートしようとする代わりに、各ボタンに異なるactionCommandを与える方が簡単だと思います。
private JDialog dialog;
private String inputValue;
String showPromptDialog(Frame parent) {
dialog = new JDialog(parent, true);
dialog.setDefaultCloseOperation(JDialog.DISPOSE_ON_CLOSE);
// [add components to dialog here]
firstButton.setAction(new ButtonAction("Button 1", "first"));
secondButton.setAction(new ButtonAction("Button 2", "second"));
thirdButton.setAction(new ButtonAction("Button 3", "third"));
fourthButton.setAction(new ButtonAction("Button 4", "fourth"));
fifthButton.setAction(new ButtonAction("Button 5", "fifth"));
sixthButton.setAction(new ButtonAction("Button 6", "sixth"));
dialog.pack();
dialog.setLocationRelativeTo(parent);
inputValue = null;
dialog.setVisible(true);
return inputValue;
}
private class ButtonAction
extends AbstractAction {
private static final long serialVersionUID = 1;
ButtonAction(String text,
String actionCommand) {
super(text);
putValue(ACTION_COMMAND_KEY, actionCommand);
}
public void actionPerformed(ActionEvent event) {
inputValue = event.getActionCommand();
dialog.dispose();
}
}