Scrollbars disappear when constructing JScrollPane with JPanel

Question

What I want to do is load an image into a JScrollPane, surrounded by other components, and when the window is smaller than the image, it should be surrounded by scrollbars so that you can view the entire image by scrolling around.

I am doing this by constructing a JScrollPane with my own class ImagePanel, which extends JPanel, which paints the image. The image is loaded and displayed correctly, but the scrollbars disappear. Is there some part of JComponents that I've misunderstood or what is wrong?

public class ImagePanel extends JPanel {
    private BufferedImage img;
    public ImagePanel () {

    }
    public void paintComponent(Graphics g) {
        super.paintComponent(g);
        g.drawImage(img, 0, 0, this);
    }
    public void setImage(BufferedImage img) {
        this.img = img;
    }
}


public class MainGUI extends JPanel {
    private ImagePanel imgP;
    private JScrollPane pane;
    public MainGUI () {
        setLayout(new GridBagLayout());

        //Create local components
        //JComponents
        JButton hideButton = new JButton("Hide");
        JButton hidecategoryButton = new JButton ("Hide Category");
        JButton removeButton = new JButton("Remove");
        JButton searchButton = new JButton("Search");
        JButton whatishereButton = new JButton("What is here?");

        JComboBox<String> placeComboBox;

        JLabel categoriesLabel = new JLabel("Categories");
        JLabel newLabel = new JLabel("New: ");

        JTextArea categoriesArea = new JTextArea();

        JTextField searchField = new JTextField(10);

        //Other
        String[] placeTypes = {"Named Place", "Described Place"};


        //Initialize and add behaviour
        searchButton.addActionListener(new ActionListener() {

            @Override
            public void actionPerformed(ActionEvent e) {
                String s = searchField.getText();

            }

        });
        categoriesArea.setEditable(false);
        placeComboBox = new JComboBox<>(placeTypes);

        searchField.setToolTipText("Search...");
        searchField.setMinimumSize(new Dimension(5, 1));

        //Just fileloading test
        BufferedImage bi2 = null;
        try {
            bi2 = ImageIO.read(new File("dafoe.jpg"));
        }
        catch (Exception ex) {

        }
        imgP = new ImagePanel();
        imgP.setImage(bi2);
        pane = new JScrollPane(imgP);

        //Add to panel
        GridBagConstraints gc;

        //Top row
        JPanel toprowPanel = new JPanel();
        toprowPanel.add(newLabel);
        toprowPanel.add(placeComboBox);
        toprowPanel.add(searchField);
        toprowPanel.add(searchButton);
        toprowPanel.add(hideButton);
        toprowPanel.add(removeButton);
        toprowPanel.add(whatishereButton);

        gc = new GridBagConstraints();
        gc.gridx = 0;
        gc.gridy = 0;
        gc.weightx = 1;
        //gc.gridwidth = 6;
        add(toprowPanel, gc);

        //Hide category
        gc = new GridBagConstraints();
        gc.anchor = GridBagConstraints.NORTH; 
        gc.gridx = 7;
        gc.gridy = 3;
        gc.weightx = 0;
        gc.weighty = 0;
        add(hidecategoryButton, gc);

        //Category Label
        gc = new GridBagConstraints();
        gc.anchor = GridBagConstraints.BELOW_BASELINE;
        gc.gridx = 7;
        gc.gridy = 1;
        gc.weightx = 0;
        gc.weighty = 0;
        add(categoriesLabel, gc);       


        //categoriesarea
        gc = new GridBagConstraints();
        gc.anchor = GridBagConstraints.BASELINE_TRAILING;
        gc.fill = GridBagConstraints.BOTH;
        gc.gridx = 7;
        gc.gridy = 2;
        gc.ipadx = 5;
        gc.ipady = 70;
        gc.weightx = 0;
        gc.weighty = 0;
        add(categoriesArea, gc);

        //Image
        gc = getImageConstraints();

        add(pane,  gc);
    }

    public void updateImage(BufferedImage bi) {
        imgP.setImage(bi);
        //imgP.repaint();

        pane.repaint();
    }
    private GridBagConstraints getImageConstraints() {
        GridBagConstraints gc = new GridBagConstraints();
        gc.fill = GridBagConstraints.BOTH;
        gc.gridheight = 3;
        gc.gridwidth = 1;
        gc.gridx = 0;
        gc.gridy = 1;
        gc.weightx = 1;
        gc.weighty = 1;
        return gc;
    }
}

Answer 1

my own class ImagePanel, which extends JPanel, which paints the image. The image is loaded and displayed correctly, but the scrollbars disappear.

The scrollbars will appear automatically when the preferred size of the component added to the scroll pane is greater than the size of the scroll pane.

Your custom component has a preferred size of (0, 0) so the scrollbars will never appear.

You need to override the getPreferredSize() of your ImagePanel class to return the size of the image.

Or even easier just use a JLabel with an ImageIcon and add the label to the scroll pane. A JLabel already implements the getPreferredSize() method correctly.