javafx how to add a listener to a toggle radiobutton

javafx set variable value based on state of radiobutton

ToggleGroup stateGroup = new ToggleGroup();
stateOn.setToggleGroup(stateGroup);
stateOff.setToggleGroup(stateGroup);
stateGroup.selectedToggleProperty().addListener(
   (observable, oldToggle, newToggle) -> {
       if (newToggle == stateOn) {
           state = 'on';
       } else if (newToggle == stateOff) {
           state = 'off';
       } else {
           state = '?';
       }
    }
);