title: My Minesweeper program written in Java is throwing a NullPointerException
body: <p>I have written a Minesweeper game in Java. However, after I click on a few buttons, it throws a <code>NullPointerException</code>. Here is the complete program (I'm afraid it is huge, and my comments won't help much, I suppose!)</p>
  
  <pre><code>import java.util.Random;
  import javax.swing.JToggleButton;
  
      /**
       * This class represents a button in a {@code MineSweeperGrid}.
       */
      @SuppressWarnings("serial") public class MineButton extends JToggleButton {
          public final boolean safe; // Whether this button hides a mine or not (true if not).
          private boolean clicked; // Whether this button has been clicked or not.
          public final int id; // The id number of this button. This can be used to determine the position of the button on
                                  // the grid.
  
          /**
           * Constructs a new {@code MineButton} with a randomly generated safety status, and a user defined id.
           * 
           * @param id
           *            The identity number of this button, which can be used to determine its position on a grid of buttons.
           */
          public MineButton(int id) {
              super();
              safe = new Random().nextBoolean();
              clicked = false;
              this.id = id;
          }
  
          public boolean hasBeenClicked() {
              return clicked;
          }
  
      }
  
  
  import java.awt.GridLayout;
  import java.awt.event.ActionEvent;
  import java.awt.event.ActionListener;
  
  import javax.swing.JOptionPane;
  import javax.swing.JPanel;
  
  @SuppressWarnings("serial") public class MineSweeperGrid extends JPanel {
      public final int size; // The size of this grid.
      private int noOfMines; // The number of mines in the grid.
      private int btnsLeft; // The buttons left to clear.
      private MineButton[][] grid; // The grid itself, which is represented as an array of MineButtons.
  
      /**
       * Creates a new MineSweeperGrid object according to the size specified as the argument.
       * @param size
       *            The length or breadth of the grid. The number of buttons in the grid will be size*size.
       */
      public MineSweeperGrid(int size) {
          super(); // Call the superclass constructor.
          if(size &lt; 3) // This is minimum limit of the size, because our code won't work properly with a size lesser than
                          // this.
              throw new IllegalArgumentException("The size of the grid cannot be less than three.");
          setLayout(new GridLayout(10, 10)); // Set the layout of this panel.
          this.size = size;
          grid = new MineButton[size][size];
          int id = noOfMines = btnsLeft = 0; // Initialize id, noOfMines &amp; btnsLeft to 0.
  
          // Initialize each button of the grid.
          for(int i = 0; i &lt; (size - 1); i++)
              for(int j = 0; j &lt; size; j++) {
                  grid[i][j] = new MineButton(id); // Construct the button.
                  if(!grid[i][j].safe)
                      noOfMines++;
                  grid[i][j].addActionListener(new MineListener()); // Add a listener to it (which does not work
                                                                      // properly).
                  grid[i][j].setEnabled(true); // Disable this button.
                  add(grid[i][j]); // Add it to the grid.
                  id++;
              }
          btnsLeft = (size * size) - noOfMines; // Set the number of buttons to be cleared .
      }
  
      /**
       * 
       * The ActionListener for the MineButtons in our grid.
       */
      private class MineListener implements ActionListener {
          /*
           * This is test code. Not working currently!
           */
          @Override public void actionPerformed(ActionEvent evt) {
              MineButton btn = (MineButton)evt.getSource(); // Get the button which generated this event.
              btn.setSelected(false);
              if(!(btn.safe)) {
                  btn.setText("M");
                  return;
              }
              btn.setEnabled(false); // Since now we know he's safe, disable this button.
              MineButton[] btns = getNeighbours(btn); // Get this button's neighbours, i.e., the
                                                      // buttons that surround it.
              int count = 0; // Initialize count to 0.
              for(int i = 0; i &lt; btns.length; i++)
                  if(!btns[i].safe)
                      count++;
              btn.setText("" + count + "");
              btnsLeft--;
              if(btnsLeft == 0) {
                  JOptionPane.showMessageDialog(null, "You have cleared all the mines! You win!");
                  System.exit(0);
  
              }
          }
  
          /**
           * Method which computes the buttons surrounding the button passed as parameter.
           * 
           * @param btn
           *            The button passed as parameter.
           * @return The buttons surrounding this button as an array of buttons.
           * @throws IllegalArgumentException
           *             If the parameter passed is null.
           */
          private MineButton[] getNeighbours(MineButton btn) {
              int x = btn.id / size, y = btn.id % size;
              System.out.println(x + ", " + y);
              if((x == 0) &amp;&amp; (y == 0)) // This is the first button in the grid, in the top left corner.
                  return new MineButton[] {grid[x][y + 1], grid[x + 1][y], grid[x + 1][y + 1]}; // This is OK.
  
              else if((x == (size - 1)) &amp;&amp; (y == (size - 1))) // This is the last button in the grid, in the lower right
                                                              // corner.
                  return new MineButton[] {grid[x - 1][y - 1], grid[x - 1][y], grid[x][y - 1]}; // This is OK.
  
              else if((x == 0) &amp;&amp; (y == (size - 1))) // This is the button in the top right corner.
                  return new MineButton[] {grid[x][y - 1], grid[x + 1][y], grid[x + 1][y - 1]};
  
              else if((x == (size - 1)) &amp;&amp; (y == 0)) // This is the button in the lower left corner.
                  return new MineButton[] {grid[x][y + 1], grid[x - 1][y], grid[x - 1][y + 1]}; // This is OK.
  
              else if(x == 0) // This is a button belonging to the first row.
                  return new MineButton[] {grid[x][y + 1], grid[x + 1][y], grid[x + 1][y + 1], grid[x + 1][y - 1],
                          grid[x][y - 1]}; // This is OK.
  
              else if(y == 0) // This is a button belonging to the first column.
                  return new MineButton[] {grid[x][y + 1], grid[x + 1][y], grid[x + 1][y + 1], grid[x - 1][y],
                          grid[x - 1][y + 1]}; // This is OK.
  
              else if(x == (size - 1)) // This is a button in the last row.
                  return new MineButton[] {grid[x][y - 1], grid[x - 1][y], grid[x - 1][y - 1], grid[x - 1][y + 1],
                          grid[x][y + 1]}; // This is OK.;
  
              else if(y == (size - 1)) // This is a button in the last column.
                  return new MineButton[] {grid[x][y - 1], grid[x - 1][y], grid[x - 1][y - 1], grid[x + 1][y],
                          grid[x + 1][y - 1]}; // This is OK.
  
              // For all other buttons.
              return new MineButton[] {grid[x + 1][y + 1], grid[x + 1][y], grid[x][y + 1], grid[x - 1][y - 1],
                      grid[x - 1][y], grid[x][y - 1], grid[x - 1][y + 1], grid[x + 1][y - 1]}; // This is OK.
  
          }
      }
  }
  
  
  import java.awt.EventQueue;
  import java.awt.event.ActionEvent;
  import java.awt.event.ActionListener;
  
  import javax.swing.JFrame;
  import javax.swing.JMenu;
  import javax.swing.JMenuBar;
  import javax.swing.JMenuItem;
  import javax.swing.JPanel;
  
  import org.mandal.data.MineSweeperGrid;
  
  public class MineSweeperUI {
  
      private JFrame frame;
      private JPanel content;
      private ButtonHandler listener;
      private MineSweeperGrid grid;
      private JMenuBar menuBar;
      private JMenu file;
      private JMenuItem newGame;
      private JMenuItem options;
      private JMenuItem close;
      private JMenu help;
      private JMenuItem instructions;
      private JMenuItem about;
  
      /**
       * 
       */
      private class ButtonHandler implements ActionListener {
          @Override public void actionPerformed(ActionEvent evt) {
          }
      }
  
      /**
       * Launch the application.
       */
      public static void main(String[] args) {
          EventQueue.invokeLater(new Runnable() {
              @Override public void run() {
                  try {
                      MineSweeperUI window = new MineSweeperUI();
                      window.frame.setVisible(true);
                  } catch(Exception e) {
                      e.printStackTrace();
                  }
              }
          });
      }
  
      /**
       * Create the application.
       */
      public MineSweeperUI() {
          initialize();
      }
  
      /**
       * Initialize the contents of the frame.
       */
      private void initialize() {
          frame = new JFrame("MineSweeper v1.00");
          frame.setResizable(false);
          frame.setBounds(100, 100, 450, 300);
          frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
  
          content = new JPanel();
          content.setLayout(null);
  
          grid = new MineSweeperGrid(10);
          grid.setBounds(0, 0, 444, 251);
          content.add(grid);
  
          menuBar = new JMenuBar();
          frame.setJMenuBar(menuBar);
  
          file = new JMenu("File");
          menuBar.add(file);
  
          newGame = new JMenuItem("New Game");
          file.add(newGame);
  
          options = new JMenuItem("Options...");
          file.add(options);
  
          close = new JMenuItem("Close");
          file.add(close);
  
          help = new JMenu("Help");
          menuBar.add(help);
  
          instructions = new JMenuItem("How to play?");
          help.add(instructions);
  
          about = new JMenuItem("About...");
          help.add(about);
  
          frame.setContentPane(content);
      }
  }
  </code></pre>
  
  <p>As you can see, the program is not yet complete, and some parts have been simplified (such as displaying "M" when you hit a mine, instead of handling the loss) for debugging purposes. This is the stack trace :</p>
  
  <pre><code>Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
      at org.mandal.data.MineSweeperGrid$MineListener.actionPerformed(MineSweeperGrid.java:68)
      at javax.swing.AbstractButton.fireActionPerformed(Unknown Source)
      at javax.swing.AbstractButton$Handler.actionPerformed(Unknown Source)
      at javax.swing.DefaultButtonModel.fireActionPerformed(Unknown Source)
      at javax.swing.JToggleButton$ToggleButtonModel.setPressed(Unknown Source)
      at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(Unknown Source)
      at java.awt.Component.processMouseEvent(Unknown Source)
      at javax.swing.JComponent.processMouseEvent(Unknown Source)
      at java.awt.Component.processEvent(Unknown Source)
      at java.awt.Container.processEvent(Unknown Source)
      at java.awt.Component.dispatchEventImpl(Unknown Source)
      at java.awt.Container.dispatchEventImpl(Unknown Source)
      at java.awt.Component.dispatchEvent(Unknown Source)
      at java.awt.LightweightDispatcher.retargetMouseEvent(Unknown Source)
      at java.awt.LightweightDispatcher.processMouseEvent(Unknown Source)
      at java.awt.LightweightDispatcher.dispatchEvent(Unknown Source)
      at java.awt.Container.dispatchEventImpl(Unknown Source)
      at java.awt.Window.dispatchEventImpl(Unknown Source)
      at java.awt.Component.dispatchEvent(Unknown Source)
      at java.awt.EventQueue.dispatchEventImpl(Unknown Source)
      at java.awt.EventQueue.access$200(Unknown Source)
      at java.awt.EventQueue$3.run(Unknown Source)
      at java.awt.EventQueue$3.run(Unknown Source)
      at java.security.AccessController.doPrivileged(Native Method)
      at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source)
      at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source)
      at java.awt.EventQueue$4.run(Unknown Source)
      at java.awt.EventQueue$4.run(Unknown Source)
      at java.security.AccessController.doPrivileged(Native Method)
      at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source)
      at java.awt.EventQueue.dispatchEvent(Unknown Source)
      at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown Source)
      at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source)
      at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source)
      at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
      at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
      at java.awt.EventDispatchThread.run(Unknown Source)
  </code></pre>