JButton array ActionListener value

Question

for(gbc.gridy = 3; gbc.gridy > 0; gbc.gridy--)
        for(gbc.gridx = 0; gbc.gridx < 3;gbc.gridx++)
          {
            grid[btnNum] = new JButton("" + (btnNum+1));
            grid[btnNum].setPreferredSize(new Dimension(75,75));
            frame.add(grid[btnNum], gbc);
            grid[btnNum].addActionListener(this);
            grid[btnNum].addKeyListener(this);
            btnNum++;
          }

I have an array of buttons displayed in a 3x3 grid and each one has an action listener.

public void actionPerformed(ActionEvent e){
        String output = "";
        if(e.getSource() == grid[0]){
            System.out.println("button 1");
        }
}

Is this not correct?

Complete example in context:

public class ButtonGrid implements ActionListener, KeyListener{

    JFrame frame=new JFrame(); //creates frame
    JButton[] grid; //names the grid of buttons




    public ButtonGrid(){ //constructor
            frame.setTitle("MPC");
            frame.setLayout(new GridBagLayout()); //set layout
            JButton[] grid=new JButton[12]; //allocate the size of grid

            GridBagConstraints gbc = new GridBagConstraints();
            gbc.gridx = 0;
            gbc.gridy = 0;
            int btnNum = 0;

            grid[9] = new JButton("0");
            grid[9].setPreferredSize(new Dimension(75,75));
            grid[10] = new JButton("-");
            grid[10].setPreferredSize(new Dimension(75,75));
            grid[11] = new JButton("=");
            grid[11].setPreferredSize(new Dimension(75,75));
            frame.add(grid[9], gbc);
            gbc.gridx++;
            frame.add(grid[10], gbc);
            gbc.gridx++;
            frame.add(grid[11], gbc);

            for(gbc.gridy = 3; gbc.gridy > 0; gbc.gridy--)
                for(gbc.gridx = 0; gbc.gridx < 3;gbc.gridx++){
                    grid[btnNum] = new JButton("" + (btnNum+1));
                    grid[btnNum].setPreferredSize(new Dimension(75,75));
                    frame.add(grid[btnNum], gbc);
                    grid[btnNum].addActionListener(this);
                    grid[btnNum].addKeyListener(this);
                    btnNum++;
                }
            frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
            frame.pack(); //sets appropriate size for frame
            frame.setVisible(true); //makes frame visible
 }
 public static void main(String[] args) {

        new ButtonGrid();//makes new ButtonGrid with 2 parameters


    }
 public void actionPerformed(ActionEvent e){
        String output = "";
        if(e.getSource() == grid[0]){
            System.out.println("button one");
            //playSound(abc.kick);
        }}

Answer 1

Your grid variable is null in the ActionListener. My bet -- you're shadowing the variable.

Solution: make sure that you initialize the grid field, not a local grid variable

public class ButtonGrid implements ActionListener, KeyListener{

    JFrame frame=new JFrame(); 
    JButton[] grid; // grid field remains null

    public ButtonGrid(){ 
            frame.setTitle("MPC");
            frame.setLayout(new GridBagLayout()); 
            JButton[] grid=new JButton[12]; //  ****** a LOCAL variable ******

At the indicated line you create the shadowed variable. This means because you redeclare grid within the constructor, you initialize only the local variable and not the field that was declared in the class leaving the field null. Solution: Don't re-declare grid. Change it to:

public class ButtonGrid implements ActionListener, KeyListener{

    JFrame frame = new JFrame(); 
    JButton[] grid; 

    public ButtonGrid(){
            frame.setTitle("MPC");
            frame.setLayout(new GridBagLayout()); 
            grid = new JButton[12]; //  ***** this initializes the field ******

Answer 2

Try this.

public void actionPerformed(ActionEvent e){
        String output = "";
   for(int i=0; i<=grid.length; i++){
       if(e.getSource() == grid[i]){
            System.out.println("button "+i);
       }
   }
}