I have written a Minesweeper game in Java. However, after I click on a few buttons, it throws a NullPointerException. Here is the complete program (I’m afraid it is huge, and my comments won’t help much, I suppose!)
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 < 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 & btnsLeft to 0.
// Initialize each button of the grid.
for(int i = 0; i < (size - 1); i++)
for(int j = 0; j < 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 < 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) && (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)) && (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) && (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)) && (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);
}
}
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 :
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)
It seems that:
should be:
otherwise you would not fill the last array
grid[size - 1]in the last iteration.grid[size - 1]refers to an empty array:or in other words: