Restrict checkboxes checked. Javafx - javafx

Question

How to restrict the number of checkboxes that a user can select? I want to make it so the user can only select 3. I think I should use isSelected, but I don't know how to link all the checkboxes.
CheckBox cb1 = new CheckBox("Pepperoni");
CheckBox cb2 = new CheckBox("Cheese");
CheckBox cb3 = new CheckBox("Tomato");
CheckBox cb4 = new CheckBox("Olives");
CheckBox cb5 = new CheckBox("Chicken");
//if 3 are already selected, it should not be possible for the user to select more

Answer 1

Simply keep track of the selected CheckBoxes using a suitable data structure (LinkedHashSet would allow you to use Set functionality and allow you to determine the first one inserted).
Updating the data structure can be done from a listener to the selected properties of the CheckBoxes.
#Override
public void start(Stage primaryStage) throws Exception {
String[] ingredients = new String[]{
"Pepperoni",
"Cheese",
"Tomato",
"Olives",
"Chicken"
};
final int maxCount = 3;
final Set<CheckBox> activeBoxes = new LinkedHashSet<>();
ChangeListener<Boolean> listener = (o, oldValue, newValue) -> {
// get checkbox containing property
CheckBox cb = (CheckBox) ((ReadOnlyProperty) o).getBean();
if (newValue) {
activeBoxes.add(cb);
if (activeBoxes.size() > maxCount) {
// get first checkbox to be activated
cb = activeBoxes.iterator().next();
// unselect; change listener will remove
cb.setSelected(false);
}
} else {
activeBoxes.remove(cb);
}
};
VBox root = new VBox();
// create checkboxes
for (int i = 0; i < ingredients.length; i++) {
CheckBox cb = new CheckBox(ingredients[i]);
cb.selectedProperty().addListener(listener);
root.getChildren().add(cb);
}
Scene scene = new Scene(root);
primaryStage.setScene(scene);
primaryStage.show();
}
Note: If you want the user to be simply unable to select more CheckBoxes when there are 3 selected, simply disable all unselected CheckBoxes when you reach 3 selected CheckBoxes:
final CheckBox[] checkBoxes = new CheckBox[ingredients.length];
ChangeListener<Boolean> listener = new ChangeListener<Boolean>() {
private int activeCount = 0;
public void changed(ObservableValue<? extends Boolean> o, Boolean oldValue, Boolean newValue) {
if (newValue) {
activeCount++;
if (activeCount == maxCount) {
// disable unselected CheckBoxes
for (CheckBox cb : checkBoxes) {
if (!cb.isSelected()) {
cb.setDisable(true);
}
}
}
} else {
if (activeCount == maxCount) {
// reenable CheckBoxes
for (CheckBox cb : checkBoxes) {
cb.setDisable(false);
}
}
activeCount--;
}
}
};
VBox root = new VBox();
// create checkboxes
for (int i = 0; i < ingredients.length; i++) {
CheckBox cb = new CheckBox(ingredients[i]);
cb.selectedProperty().addListener(listener);
root.getChildren().add(cb);
checkBoxes[i] = cb;
}