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?

Answer 1

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.