Why do I suddenly need to declare a variable as "final" when adding an ActionListener to a button?

Question

Possible Duplicate:
Why are only final variables accessible in anonymous class?

This is the code I have for a "random math game" I'm creating for class:

package RandomMathGame;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.*;

public class RandomMathGame {

    public static void main(String[] args) {
        RandomProblemGenerator randomProblems = new RandomProblemGenerator(10);
        final int numberProblems = 10;
        int correctScore = 0;
        JPanel panel = new JPanel();
       int answer;
        int correctAnswer;
        JLabel[] mathProblems = new JLabel[numberProblems];
        final JTextField[] mathAnswers = new JTextField[numberProblems];
        JLabel[] correctYesNo = new JLabel[numberProblems];
        final JLabel score = new JLabel(correctScore + "/10");
        JButton submit = new JButton("Submit");
        for (int i = 1; i <= numberProblems; i++)
        {
            final int X = randomProblems.createNumberX();
            final int Y = randomProblems.createNumberY();

            mathProblems[i] = new JLabel("" + X + " * " + Y + " = ");
            mathAnswers[i] = new JTextField();


             answer = Integer.parseInt(mathAnswers[i].getText());
             correctAnswer = X * Y;

            if (answer == correctAnswer)
            {
                correctYesNo[i] = new JLabel("Correct answer; good job!");
                correctScore = correctScore + 1;
            }
            else
            {
               correctYesNo[i] = new JLabel("Incorrect answer; try again!");

            }
             panel.add(mathProblems[i]);
             panel.add(mathAnswers[i]);
             panel.add(correctYesNo[i]);
            }
      submit.addActionListener(new ActionListener() {

            @Override
            public void actionPerformed(ActionEvent e) {
                score.setText("Score: " + correctScore + "/10");
            }
        });



        panel.add(submit);
        panel.add(score);

        JFrame gameFrame = new JFrame();
        gameFrame.setTitle("Random Math Game");
        gameFrame.setSize(150, 150);
        gameFrame.setVisible(true);
        gameFrame.setContentPane(panel);



        }
  }

I'm getting an error that the correctScore variable has to be declared as final before it can be used by the ActionListener. But when I set correctScore as a final, it causes all kinds of other errors. Can anybody think of a way to resolve this?

See http://stackoverflow.com/a/4732617/597657 – Eng.Fouad Jul 09 '12 at 21:33 — Eng.Fouad, Jul 09 '12 at 21:33

score 3 · Accepted Answer · answered Jul 09 '12 at 21:32

To pass a local variable to an anonymous class, it has to be final - that is part of the Java language. Now when a primitive variable is final, you can't modify it, hence the issues you get when making correctScore final.

A workaround is to use a temporary variable right before the addActionListener call:

final int temp = correctScore;

and use that temp variable in your actionPerformed method.

Note that when you do that, any future changes to correctScore will not be reflected in the value of temp.

La bla bla · Answer 2 · 2012-07-09T21:46:22.497

0

In Java, If you want to access a local variable from within a local class (

new ActionListener() {
...
...
}

is a new local anonymous class) the variable must be declared as final.

Local classes cannot access local variable of the enclosing method without final modifier for example:

public class Main {
    private int b=5;
    public void bla() {
    int a = 5;
    ActionListener al = new ActionListener() {

        @Override
        public void actionPerformed(ActionEvent arg0) {
            a = 4; // Error. a must be final
            b = 5; // OK!
        }
    };
  }
}

A possible solution to your problem is using variables declared outside of any methods (b in the above example)

edited Jul 09 '12 at 21:46

answered Jul 09 '12 at 21:31

La bla bla

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As written, that's not true. I access non-final variables from the enclosing class all the time, in cases just like this: an ActionListener updating values in controls. Maybe there's a piece you've left out that would make this statement true. – Jay Jul 09 '12 at 21:37
Sorry, I miss wrote that. class variables, can be access (i.e members - private, public etc). local variable of the method containing the anonymous class, must be final. editing my answer – La bla bla Jul 09 '12 at 21:43

Jay · Answer 3 · 2012-07-10T07:33:49.140

-1

You are getting that message because you have declared JLabel score to be final. You can't use a non-final variable to set a value for a final variable. I don't know why you want to make score final. Just remove that final.

Edit: I missed that correctScore is a function variable on my first reading. If you make it a class variable I think many of your problems will go away.

edited Jul 10 '12 at 07:33

answered Jul 09 '12 at 21:34

Jay

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Why do I suddenly need to declare a variable as "final" when adding an ActionListener to a button?

3 Answers3