Question

I have been looking for a way to display images on a JPanel in a grid-like fashion, and came across this solution. The code works very well but when I attempted to modify it by using jpg images instead of drawing rectangles, I could’t get it to work. My modification is as follows:

/*
 * courtesy of https://stackoverflow.com/questions/7222988/java-drag-and-drop-images-  in-a-list
 */

import java.awt.*;
import java.awt.datatransfer.DataFlavor;
import java.awt.datatransfer.Transferable;
import java.awt.datatransfer.UnsupportedFlavorException;
import java.io.IOException;
import java.io.Serializable;
import javax.swing.*;
import javax.swing.border.Border;

public class ImagePreviewList {

public static void main(String[] args) throws Exception {
    UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
    JFrame frame = new JFrame("Image panel");
    frame.setSize(800, 300);
    frame.setLocationByPlatform(true);
    JList imageList = createImageList();
    frame.getContentPane().add(new JScrollPane(imageList));
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    frame.setVisible(true);
}

private static JList createImageList() {

    JList imageList = new JList(createModel());
    imageList.setCellRenderer(new ImageCellRenderer());
    imageList.setLayoutOrientation(JList.HORIZONTAL_WRAP);
    imageList.setVisibleRowCount(-1);
    imageList.setSelectionMode(ListSelectionModel.SINGLE_SELECTION);
    imageList.setFixedCellWidth(240);
    imageList.setFixedCellHeight(120);

    imageList.setDragEnabled(true);
    imageList.setDropMode(DropMode.INSERT);
    imageList.setTransferHandler(new ImageTransferHandler(imageList));

    return imageList;
}

private static DefaultListModel createModel() {
    DefaultListModel model = new DefaultListModel();
    model.addElement(new RTS_image("3-Way_Stop_sign"));
    model.addElement(new RTS_image("4-Way_Stop_sign"));
    model.addElement(new RTS_image("No_Entry_sign"));
    model.addElement(new RTS_image("One-Way_Roadway_sign-0"));
    model.addElement(new RTS_image("One-Way_Roadway_sign-1"));
    model.addElement(new RTS_image("One-Way_Roadway_sign-3"));
    model.addElement(new RTS_image("Pedestrian-Priority_sign"));
    model.addElement(new RTS_image("Stop#Go_sign"));
    model.addElement(new RTS_image("Stop_and_Yield_sign"));
    model.addElement(new RTS_image("Stop_sign-0"));
    model.addElement(new RTS_image("Stop_sign-1"));
    model.addElement(new RTS_image("Yield_at_Mini-Circle_sign"));
    model.addElement(new RTS_image("Yield_sign"));
    model.addElement(new RTS_image("Yield_to_Pedestrians_sign"));
    model.addElement(new RTS_image("Yield_to_oncoming_traffic_sign"));
    return model;
}

static class ImageTransferHandler extends TransferHandler {

    private static final DataFlavor DATA_FLAVOUR = new DataFlavor(RTS_image.class,  "Images");

    private final JList previewList;
    private boolean inDrag;

    ImageTransferHandler(JList previewList) {
        this.previewList = previewList;
    }

    public int getSourceActions(JComponent c) {
        return TransferHandler.MOVE;
    }

    protected Transferable createTransferable(JComponent c) {
        inDrag = true;
        return new Transferable() {
            public DataFlavor[] getTransferDataFlavors() {
                return new DataFlavor[] {DATA_FLAVOUR};
            }

            public boolean isDataFlavorSupported(DataFlavor flavor) {
                return flavor.equals(DATA_FLAVOUR);
            }

            public Object getTransferData(DataFlavor flavor) throws UnsupportedFlavorException, IOException {
                return previewList.getSelectedValue();
            }
        };
    }

    public boolean canImport(TransferSupport support) {
        if (!inDrag || !support.isDataFlavorSupported(DATA_FLAVOUR)) {
            return false;
        }

        JList.DropLocation dl = (JList.DropLocation)support.getDropLocation();
        if (dl.getIndex() == -1) {
            return false;
        } else {
            return true;
        }
    }

    public boolean importData(TransferSupport support) {
        if (!canImport(support)) {
            return false;
        }

        Transferable transferable = support.getTransferable();
        try {
            Object draggedImage = transferable.getTransferData(DATA_FLAVOUR);

            JList.DropLocation dl = (JList.DropLocation)support.getDropLocation();
            DefaultListModel model = (DefaultListModel)previewList.getModel();
            int dropIndex = dl.getIndex();
            if (model.indexOf(draggedImage) < dropIndex) {
                dropIndex--;
            }
            model.removeElement(draggedImage);
            model.add(dropIndex, draggedImage);
            return true;
        } catch (Exception e) {
            e.printStackTrace();
            return false;
        }
    }

    protected void exportDone(JComponent source, Transferable data, int action) {
        super.exportDone(source, data, action);
        inDrag = false;
    }
}

static class ImageCellRenderer extends JPanel implements ListCellRenderer {

    DefaultListCellRenderer defaultListCellRenderer = new DefaultListCellRenderer();
    JLabel imageLabel = new JLabel();
    JLabel descriptionLabel = new JLabel();

    ImageCellRenderer() {
        setLayout(new BorderLayout());
        Border emptyBorder = BorderFactory.createEmptyBorder(5, 5, 5, 5);
        imageLabel.setBorder(emptyBorder);
        descriptionLabel.setBorder(emptyBorder);
        add(imageLabel, BorderLayout.CENTER);
        add(descriptionLabel, BorderLayout.SOUTH);
    }

    public Component getListCellRendererComponent(JList list, Object value, int index, boolean isSelected, boolean cellHasFocus) {
        defaultListCellRenderer.getListCellRendererComponent(list, value, index, isSelected, cellHasFocus);
        setBorder(defaultListCellRenderer.getBorder());
        setBackground(defaultListCellRenderer.getBackground());
        imageLabel.setIcon((Icon)value);
        descriptionLabel.setText("Description");
        return this;
    }
}

static class RTS_image implements Icon, Serializable {
    private String Name_of_RoadSign; 

    RTS_image(String name_of_roadsign) {
        this.Name_of_RoadSign = name_of_roadsign;
    }

    java.net.URL imgURL = com.umusebo.RTS.ImagePreviewList.class.getResource("images/01_Regulatory_Signs/01_Control_Signs/reg_cont_" + Name_of_RoadSign + ".jpg");

    Image img = Toolkit.getDefaultToolkit().createImage(imgURL);

    public void paintIcon(Component c, Graphics g, int x, int y) {
        g.drawImage(img, x, y, c);
    }

    public int getIconWidth() {
        return 114;
    }

    public int getIconHeight() {
        return 125;
    }

    public String getName_of_RoadSign() {
        return Name_of_RoadSign;
    }

    public boolean equals(Object o) {
        if (o == null || getClass() != o.getClass()) {
            return false;
        }
        return Name_of_RoadSign.equals(((RTS_image)o).Name_of_RoadSign);
    }

}

}

Upon execution I get the following errors (which repeats 15 times for the 15 images I’m trying to display):

Uncaught error fetching image:
java.lang.NullPointerException
at sun.awt.image.URLImageSource.getConnection(URLImageSource.java:115)
at sun.awt.image.URLImageSource.getDecoder(URLImageSource.java:125)
at sun.awt.image.InputStreamImageSource.doFetch(InputStreamImageSource.java:263)
at sun.awt.image.ImageFetcher.fetchloop(ImageFetcher.java:205)
at sun.awt.image.ImageFetcher.run(ImageFetcher.java:169)

Where could the problem be?

Answer 1

You’ve declared a number of class level field that you are trying to initialise using the values from other variables that are null

For example, you are declaring and intialising imgURL from the variable Name_of_RoadSign, which will be null.

java.net.URL imgURL = com.umusebo.RTS.ImagePreviewList.class.getResource(
    "images/01_Regulatory_Signs/01_Control_Signs/reg_cont_"
    + Name_of_RoadSign + ".jpg");

You have to remember that the classes fields are initialised BEFORE any of the classes constructors are executed.

Try initialising these class variables within constructor instead

RTS_image(String name_of_roadsign) {
    this.Name_of_RoadSign = name_of_roadsign;
    imgURL = com.umusebo.RTS.ImagePreviewList.class.getResource(
        "images/01_Regulatory_Signs/01_Control_Signs/reg_cont_"
        + Name_of_RoadSign + ".jpg");
    img = Toolkit.getDefaultToolkit().createImage(imgURL);
}

ToolKit#createImage tends to silently ignore errors and will return a none null value even if the image can’t be loaded. You are much better of using ImageIO instead. Apart from the fact that it supports a greater range of image formats, it will also throw more meaningful exceptions, making it easier to diagnose problems.

For example…

img = ImageIO.read(imgURL);

Don’t forget, this will throw an IOException so you either need to catch it or (better still) pass it back to the caller.

I would also encourage you to familiarise yourself with the Code Conventions for the Java Programming Language. You won’t make any friends or influence people by ignoring them. 😉