Multiple, but limited, CheckBoxes in FXML/JavaFX - checkbox

Question

I'm trying to find a way to use check boxes to allow the user to pick two things from a list of three, and then have the remaining check box become disabled until one of the others is un-selected. I have a ChangeListener attached to all three check boxes, so I can register when two have been selected, but I don't know how to target the "other" box/boxes to disable/enable them.
ChangeListener checkboxlistener = new ChangeListener() {
int i = 0;
#Override
public void changed(ObservableValue observable, Object oldValue, Object newValue) {
if((boolean) observable.getValue() == true) {
i++;
System.out.println(i);
} else {
i--;
System.out.println(i);
}
if(i == 2) {
//What should I put here, if anything?//
}
}
};
checkbox1.selectedProperty().addListener(checkboxlistener);
checkbox2.selectedProperty().addListener(checkboxlistener);
checkbox3.selectedProperty().addListener(checkboxlistener);
}

Answer 1

Keep a collection of selected check boxes and unselected check boxes. Add a listener to each check box's selectedProperty and make sure it is in the correct collections when the selection changes.
Then you can just observe the size of the collection of selected check boxes, and update the disable state of all the unselected check boxes accordingly.
Here's an example:
MultipleCheckBoxExampleController.java:
package multiplecheckbox;
import javafx.beans.binding.Bindings;
import javafx.beans.binding.IntegerBinding;
import javafx.collections.FXCollections;
import javafx.collections.ObservableSet;
import javafx.fxml.FXML;
import javafx.scene.control.Button;
import javafx.scene.control.CheckBox;
public class MultipleCheckBoxExampleController {
#FXML
private CheckBox checkBox1 ;
#FXML
private CheckBox checkBox2 ;
#FXML
private CheckBox checkBox3 ;
#FXML
private Button submitButton ;
private ObservableSet<CheckBox> selectedCheckBoxes = FXCollections.observableSet();
private ObservableSet<CheckBox> unselectedCheckBoxes = FXCollections.observableSet();
private IntegerBinding numCheckBoxesSelected = Bindings.size(selectedCheckBoxes);
private final int maxNumSelected = 2;
public void initialize() {
configureCheckBox(checkBox1);
configureCheckBox(checkBox2);
configureCheckBox(checkBox3);
submitButton.setDisable(true);
numCheckBoxesSelected.addListener((obs, oldSelectedCount, newSelectedCount) -> {
if (newSelectedCount.intValue() >= maxNumSelected) {
unselectedCheckBoxes.forEach(cb -> cb.setDisable(true));
submitButton.setDisable(false);
} else {
unselectedCheckBoxes.forEach(cb -> cb.setDisable(false));
submitButton.setDisable(true);
}
});
}
private void configureCheckBox(CheckBox checkBox) {
if (checkBox.isSelected()) {
selectedCheckBoxes.add(checkBox);
} else {
unselectedCheckBoxes.add(checkBox);
}
checkBox.selectedProperty().addListener((obs, wasSelected, isNowSelected) -> {
if (isNowSelected) {
unselectedCheckBoxes.remove(checkBox);
selectedCheckBoxes.add(checkBox);
} else {
selectedCheckBoxes.remove(checkBox);
unselectedCheckBoxes.add(checkBox);
}
});
}
}
MultipleCheckBoxExample.fxml:
<?xml version="1.0" encoding="UTF-8"?>
<?import javafx.scene.layout.VBox?>
<?import javafx.scene.control.CheckBox?>
<?import javafx.scene.control.Button?>
<?import javafx.geometry.Insets?>
<VBox xmlns:fx="http://javafx.com/fxml/1" fx:controller="multiplecheckbox.MultipleCheckBoxExampleController"
alignment="center" spacing="5">
<padding>
<Insets top="10" bottom="10" left="10" right="10" />
</padding>
<CheckBox fx:id="checkBox1" text="Choice 1" />
<CheckBox fx:id="checkBox2" text="Choice 2" />
<CheckBox fx:id="checkBox3" text="Choice 3" />
<Button fx:id="submitButton" text="Submit" />
</VBox>
Main.java:
package multiplecheckbox;
import javafx.application.Application;
import javafx.stage.Stage;
import javafx.scene.Scene;
import javafx.scene.layout.VBox;
import javafx.fxml.FXMLLoader;
public class Main extends Application {
#Override
public void start(Stage primaryStage) {
try {
VBox root = FXMLLoader.load(getClass().getResource("MultipleCheckBoxExample.fxml"));
Scene scene = new Scene(root,400,400);
primaryStage.setScene(scene);
primaryStage.show();
} catch(Exception e) {
e.printStackTrace();
}
}
public static void main(String[] args) {
launch(args);
}
}