Question

I am trying to take an image and turn it into tiles that are laid out in GridLayout, but receive a runtime error.

Here is the code:

public class TileList extends JPanel {

private static final int width = 16;            //width of a tile
private static final int height = width;
private int col = 1;
private int row = 1;

private BufferedImage image;
File tilesetImage = new File("image.png");
BufferedImage tileset[];

public void loadAndSplitImage (File loadImage) {
    try{
        image = ImageIO.read(loadImage);
    }catch(Exception error) {
        System.out.println("Error: cannot read tileset image.");
    }// end try/catch
    col = image.getWidth()/width;
    row = image.getHeight()/height;
    BufferedImage tileset[] = new BufferedImage[col*row];
}// end loadAndSplitImage

public TileList() {
    loadAndSplitImage(tilesetImage);
    setLayout(new GridLayout(row,col,1,1));
    setBackground(Color.black);

    int x=0;
    int y=0;
    for (int i = 0; i < (col*row); i++) {
        JPanel panel = new JPanel();
        tileset[i] = new BufferedImage(width, height, image.getType());  //first error
        tileset[i] = image.getSubimage(x,y,x+width,y+height);
        panel.add(new JLabel(new ImageIcon(tileset[i])));
        add(panel);
        x+=width;
        y+=height;
    }// end for loop
}// end constructor

public static void createAndShowGui() {
    JFrame frame = new JFrame("TileList");
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    frame.getContentPane().add(new TileList());      //second error
    frame.pack();
    frame.setLocationRelativeTo(null);
    frame.setVisible(true);
}//end createAndShowGui

public static void main (String [] args) {
    SwingUtilities.invokeLater(new Runnable() {
        public void run () {
            createAndShowGui();           //third error
        }// end run
    });// end invokeLater
  }// end main
}// end class

This is the error I receive:

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
    at TileList.<init>(TileList.java:55)
    at TileList.createAndShowGui(TileList.java:73)
    at TileList$1.run(TileList.java:82)
    at java.awt.event.InvocationEvent.dispatch(InvocationEvent.java:251)
    at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:721)
    at java.awt.EventQueue.access$200(EventQueue.java:103)
    at java.awt.EventQueue$3.run(EventQueue.java:682)
    at java.awt.EventQueue$3.run(EventQueue.java:680)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDo
main.java:76)
    at java.awt.EventQueue.dispatchEvent(EventQueue.java:691)
    at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThre
ad.java:244)
    at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.
java:163)
    at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThre
ad.java:151)
    at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:147)

    at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:139)

    at java.awt.EventDispatchThread.run(EventDispatchThread.java:97)

What does this error mean and how did I write the code wrong?

Edit: I changed my code as Hovercraft suggested, however now I have new errors:

tileset[] = new BufferedImage[col*row];
       ^                           error: not a statement

tileset[] = new BufferedImage[col*row];
         ^                         error: ';' expected

tileset[] = new BufferedImage[col*row];
                             ^     error: not a statement

Answer 1

Your error is that you are shadowing a variable — you’re re-declaring the variable tileSet inside constructor when it was already declared in the class. Thus the tileSet variable declared in the class is never initialized since only the local variable, the one declared in the constructor is initialized.

The solution: Only declare tileSet once, in the class.

Thus, don’t do this:

public class TileList extends JPanel {

  //.... deleted for brevity

  BufferedImage tileset[];

  public void loadAndSplitImage (File loadImage) {
    try{
        image = ImageIO.read(loadImage);
    }catch(Exception error) {
        System.out.println("Error: cannot read tileset image.");
    }// end try/catch
    col = image.getWidth()/width;
    row = image.getHeight()/height;
    BufferedImage tileset[] = new BufferedImage[col*row]; // *** re-declaring variable!
  }

but instead do this:

public class TileList extends JPanel {

  //.... deleted for brevity

  BufferedImage tileset[];

  public void loadAndSplitImage (File loadImage) {
    try{
        image = ImageIO.read(loadImage);
    }catch(Exception error) {
        System.out.println("Error: cannot read tileset image.");
    }// end try/catch
    col = image.getWidth()/width;
    row = image.getHeight()/height;

    // ************
    // BufferedImage tileset[] = new BufferedImage[col*row]; // *****
    tileset[] = new BufferedImage[col*row]; // **** note the difference? ****
  }

Note that even more important than the solution for this isolated problem is to gain an understanding on how to solve NPE (NullPointerExceptions). The key is to check all the variables on the line throwing the exception, check to see which one(s) is null, and then look back to see where one of them is not being initialized properly before you try using it.