Java - Filling an array of JLabels with ImageIcons via loop(NullPointerException)

Question

the basic aim is to have a JPanel filled with 9 white squares in a 3x3 pattern; The squares are 150x150 blank white .jpg files. It must be this way since later on, the program will have to change the blank squares to one of a selection of simple images, and must be able to change any square at any time. The problem, simply, is I'm getting a NullPointerException. I have to assume it's something to do with initialising the array as null but NetBeans(yes, NetBeans...) seems to get angry with me if I don't do that. Same if I try to declare the size of the array. (That would be... "ArrayType[arraysize] arrayName;", yes?"

Egh, I'm just guessing wildly.

Edit - NullPointerException fixed, but now the blank(white) images are simply not appearing in the frame . Code below edited to reflect its new state, more potentially relevant lines added.

Here be all relevant code:

JFrame controller = new JFrame("SmartHome Interface");
controller.setVisible(true);
controller.setSize(480,500);
controller.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

//[...]

JPanel labelPanel = new JPanel();

//[...]

labelPanel.setBackground(Color.GREEN);

//[...]

ImageIcon blank = new ImageIcon("../Images/blank.jpg");

//[...]

controller.add(labelPanel);

//[...]

JLabel[] labels = new JLabel[9];
        for (int i = 0; i <= 8; i++)
        {
            int xLowBound;
            int xUpBound;
            int yLowBound; 
            int yUpBound;

            //Maths for positioning the labels correctly. Should be 150px in size with 10px gaps each.
            xLowBound = (i % 3) * 160;
            xUpBound = xLowBound + 150;
            yLowBound = (i / 3) * 160;
            yUpBound = yLowBound + 150;

            labels[i] = new JLabel();
            labels[i].setIcon(blank);
            labels[i].setBounds(xLowBound, yLowBound, xUpBound, yUpBound);
            labelPanel.add(labels[i]);
        }

Also.....is the filepath for the ImageIcon correct? The code itself being located in "src/smarthome" and the images in "src/Images"

And apologies if I broke any forum conventions/codes of conduct/etc. Newby here, tried to be careful not to but I may have forgotten something.

Answer 1

Your problem reduces to this:

JLabel[] labels = null;
for (int i = 0; i <= 8; i++) {
    labels[i].setIcon(blank);
}

This code fragment will fail because labels == null. Therefore labels[i] == null.

Use this instead:

JLabel[] labels = new JLabel[9];
for (int i = 0; i <= 8; i++) {
    labels[i] = new JLabel();
    labels[i].setIcon(blank);
}

Answer 2

---

Your filepath for imageIcons is incorrect. You should use:

ImageIcon img = new ImageIcon(getClass().getResource("../Images/blank.jpg"));    

if your code is in static method use this:

ImageIcon img = new ImageIcon(YourClass.class.getResource("../Images/blank.jpg"));

There is a good answer about loading image icons(thanks to nIcE cOw ).

---

You should call setVisible() and setSize() after adding all components to the frame.

---

Add components to frame's content pane( frame.getContentPane() ).

---

You always should place your GUI code in separate thread.

---

So, your code will be:

SwingUtilities.invokeLater(new Runnable()
{

    @Override
    public void run()
    {
        JFrame controller = new JFrame("SmartHome Interface");
        controller.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        JPanel labelPanel = new JPanel();

        labelPanel.setBackground(Color.GREEN);

        // !!!
        ImageIcon blank = new ImageIcon(YourClass.class
                .getResource("../Images/blank.jpg"));

        // !!!
        controller.getContentPane().add(labelPanel);

        JLabel[] labels = new JLabel[9];
        for (int i = 0; i <= 8; i++)
        {
            int xLowBound;
            int xUpBound;
            int yLowBound;
            int yUpBound;

            xLowBound = (i % 3) * 160;
            xUpBound = xLowBound + 150;
            yLowBound = (i / 3) * 160;
            yUpBound = yLowBound + 150;

            labels[i] = new JLabel();
            labels[i].setIcon(blank);
            labels[i].setBounds(xLowBound, yLowBound, xUpBound,
                    yUpBound);
            labelPanel.add(labels[i]);
        }

        // !!!    
        controller.setVisible(true);
        controller.setSize(480, 500);

    }
});