Java Swing: setting variables inside actionlistener that were defined outside

Question

I am new to using swing and I am having trouble with action listeners. I have an action listener set up for when a button is pressed and I am getting an error message when I try to set the value of a variable defined outside the listener. Here is the code

    // create puzzle object and array for puzzle numbers
    final Puzzle puzzle = new Puzzle();
    final int[][] nums = new int[9][9];

    // create buttons
    JButton btnSolve = new JButton("Solve");
    btnSolve.setEnabled(false);
    JButton btnGenerate = new JButton("Generate");

    // When generate button is clicked
    btnGenerate.addActionListener(new ActionListener() {
        public void actionPerformed(ActionEvent arg0) {

            // generate puzzle
            nums = puzzle.generate();

            // fill board with puzzle
            fillBoard(nums);

            // enable solve button
            btnSolve.setEnabled(true);
        }
    });

    // When solve button is clicked
    btnSolve.addActionListener(new ActionListener() {
        public void actionPerformed(ActionEvent arg0) {

            // solve the puzzle
            nums = puzzle.solve(nums);

            // fill board with solved puzzle
            fillBoard(nums);
        }
    });

The code works if I declare the nums and puzzle objects inside the btnGenerate listener without them being final. But I want to use the nums array in the btnSolve listener.

On the lines nums = puzzle.generate(); and nums = puzzle.solve(nums); I am getting the error:

The final local variable nums cannot be assigned, since it is defined in an enclosing type

What does it mean by "defined in an enclosing type" and how should this be done?

Thanks in advance.

Answer 1

Declare this final int[][] nums = new int[9][9]; as a class member and not final . The message is clear. You cannot modify something that is final . You may or may not also need to do the same with Puzzle

public class MyClass {

   int[][] nums = new int[9][9];   <-- class member instead of in constructor
   Puzzle puzzle = new Puzzle();

   public MyClass(){
   }
}