Exception occurred when iterating though for loop that includes mouseEvent action

Question

the following exception message appears when I iterate using NORMAL for-loop that includes mouseEvent action through radioButtons.

Message: Exception in thread "JavaFX Application Thread" java.lang.ArrayIndexOutOfBoundsException

But when I iterate using for-each loop, there is no problem!

please help me, thank you

Concerned code part:

 HBox p1 = new HBox();
       
        RadioButton Red = new RadioButton("RED");
        RadioButton Blue = new RadioButton("Blue");
        RadioButton Black = new RadioButton("Black");
        
          ToggleGroup tg = new ToggleGroup(); 
        
          Red.setToggleGroup(tg);
        Blue.setToggleGroup(tg);
         Black.setToggleGroup(tg);
         
        
        RadioButton [] array = {Red,Blue,Black};
        
        p1.getChildren().addAll(Red,Blue,Black);
 
     
     i = 0;
     for(i = 0; i< array.length; i++){
         
         array[i].setOnAction(e->{
         
             System.out.println(array[i].getText());
         });
         
     }

Answer 1

I'm guessing, because your question is incomplete, that you made the loop index i an instance variable, for some reason. I.e. you have something like:

import javafx.application.Application;
import javafx.scene.Scene;
import javafx.scene.control.RadioButton;
import javafx.scene.control.ToggleGroup;
import javafx.scene.layout.HBox;
import javafx.stage.Stage;

public class Demo extends Application {
    
    // Guessing:
    private int i ;

    @Override
    public void start(Stage primaryStage) throws Exception {
        HBox p1 = new HBox();

        RadioButton red = new RadioButton("RED");
        RadioButton blue = new RadioButton("Blue");
        RadioButton black = new RadioButton("Black");

        ToggleGroup tg = new ToggleGroup();

        red.setToggleGroup(tg);
        blue.setToggleGroup(tg);
        black.setToggleGroup(tg);

        RadioButton[] array = { red, blue, black };

        p1.getChildren().addAll(red, blue, black);

        i = 0;
        for (i = 0; i < array.length; i++) {

            array[i].setOnAction(e -> {

                System.out.println(array[i].getText());
            });

        }
        
        Scene scene = new Scene(p1, 400, 250);
        primaryStage.setScene(scene);
        primaryStage.show();
    }

    public static void main(String[] args) {
        Application.launch(args);
    }

}

Here i is a property of the Demo instance (i.e. the application instance on which start() is invoked). In the start() method, i is initialized to 0 and then incremented each time you iterate the for loop. The for loop exits when i is no longer less than array.length (which is 3), so when the for loop exits, i==3.

Consequently, when you press one of the buttons, the code

System.out.println(array[i].getText());

is executed. The value of i hasn't changed since the completion of the for loop, so this is equivalent to

System.out.println(array[3].getText());

and this throws an IndexOutOfBoundsException, because the value indexes in the array are 0, 1, and 2. Indeed, the complete error message is

Exception in thread "JavaFX Application Thread" java.lang.ArrayIndexOutOfBoundsException: Index 3 out of bounds for length 3

Instead, you need to use a local variable for the index in the loop:

    for (int i = 0; i < array.length; i++) {

        array[i].setOnAction(e -> { /* ... */ });

    }

The problem now is that the lambda expression in the event handler cannot access the local variable i because it is not final (or "effectively final"). The solution to this is to "capture" the value of i on each iteration of the loop in a final variable:

    for (int i = 0; i < array.length; i++) {
        final int index = i ;
        array[i].setOnAction(e -> {
            System.out.println(array[index].getText());
        });

    }

Of course, you don't actually need the index; you just need the element of the array, so you could capture that instead:

    for (int i = 0; i < array.length; i++) {
        final RadioButton button = array[i] ;
        button.setOnAction(e -> {
            System.out.println(button.getText());
        });
    }

This code is completely identical (in the sense that the compiler converts the following to the same thing) to:

    for (RadioButton button : array) {
        button.setOnAction(e -> {
            System.out.println(button.getText());
        });
    }

which is by far the preferred form for a for loop.

Here's a fully-cleaned and working version of the code:

import javafx.application.Application;
import javafx.scene.Scene;
import javafx.scene.control.RadioButton;
import javafx.scene.control.ToggleGroup;
import javafx.scene.layout.HBox;
import javafx.stage.Stage;

public class Demo extends Application {

    @Override
    public void start(Stage primaryStage) throws Exception {
        HBox p1 = new HBox();

        RadioButton red = new RadioButton("RED");
        RadioButton blue = new RadioButton("Blue");
        RadioButton black = new RadioButton("Black");

        ToggleGroup tg = new ToggleGroup();
        RadioButton[] buttons = { red, blue, black };

        tg.getToggles().setAll(buttons);
        p1.getChildren().addAll(buttons);

        for (RadioButton button : buttons) {
            button.setOnAction(e -> {
                System.out.println(button.getText());
            });
        }
        
        Scene scene = new Scene(p1, 400, 250);
        primaryStage.setScene(scene);
        primaryStage.show();
    }

    public static void main(String[] args) {
        Application.launch(args);
    }

}