垂直滚动条未出现

Question

I just began Swing. I'm trying to make a gallery app. I'm getting images that I manually imported and I display them. Depending on the amount of columns I put in parameters, the images' dimensions are calculated to be displayed properly. But, after a certain amount of rows, I want a scrollbar that could scroll and show the rest of images.

The images are displayed properly, like I wished but I tried to implement the scrollbar and it isn't appearing.

Could you tell me what's wrong in my code ?

    GUI(String title, int width, int height, int columns) {
        this.frame = new JFrame();
        this.frameWidth = width;
        this.columns = columns;

        // set Frame's size
        frame.setSize(width, height);

        // set Frame's action on close button clicked
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        // set Frame's title
        frame.setTitle(title);
        frame.setResizable(false);

        // set Frame's position at screen's center
        Dimension dim = Toolkit.getDefaultToolkit().getScreenSize();
        frame.setLocation(dim.width / 2 - frame.getSize().width / 2, (dim.height / 2 - frame.getSize().height / 2));

        panel = new JPanel();
        frame.setLayout(new GridLayout());

        this.scrollBar = new JScrollPane();
        scrollBar.setVerticalScrollBarPolicy(JScrollPane.VERTICAL_SCROLLBAR_ALWAYS);
        frame.getContentPane().add(scrollBar, BorderLayout.EAST);

        displayImages();

        frame.setContentPane(panel);
    }

displayImages() :

    private void displayImages() {
        for (File currentImage : getImagesList()) {
            // 5 is flowlayout's default borders
            // 2 because we got left and right borders
            int totalBorders = (columns * 5) * 2;
            int buttonWidth = (frameWidth - totalBorders) / columns;

            ImageIcon image = new ImageIcon(new ImageIcon(currentImage.getAbsolutePath()).getImage().getScaledInstance(buttonWidth - 20, buttonWidth - 50, Image.SCALE_DEFAULT));
            JButton button = new JButton(currentImage.getName());

            button.setPreferredSize(new Dimension(buttonWidth, buttonWidth));
            button.setIcon(image);
            button.setHorizontalTextPosition(AbstractButton.CENTER);
            button.setVerticalTextPosition(AbstractButton.BOTTOM);

            button.addActionListener(e -> onImageClicked(currentImage.getName()));
            panel.add(button);
        }
    }

Thanks, best regards

Answer 1

    panel = new JPanel();
    frame.setLayout(new GridLayout());

    this.scrollBar = new JScrollPane();
    scrollBar.setVerticalScrollBarPolicy(JScrollPane.VERTICAL_SCROLLBAR_ALWAYS);
    frame.getContentPane().add(scrollBar, BorderLayout.EAST);

    displayImages();

    frame.setContentPane(panel);

The above code is mostly wrong. You need to:

1. set the layout of the panel and add the buttons to the panel
2. add the panel containing the buttons to the scroll pane

The basic code should be something like:

//panel = new JPanel( new GridLayout(0, 1) );
layout = new GridLayout(0, 1);
panel = new JPanel( layout );
displayImages();
//frame.setLayout(new GridLayout());
this.scrollBar = new JScrollPane(panel);
scrollBar.setVerticalScrollBarPolicy(JScrollPane.VERTICAL_SCROLLBAR_ALWAYS);
frame.add(scrollBar, BorderLayout.EAST);
//displayImages();
//frame.setContentPane(panel);

Also, there is no need to play with the sizes of each component on the panel. The GridLayout will make all the buttons the same size.

Edit:

When you want to increase the columns in the GridLayout you then just do:

layout.setColumns( layout.getColumns() + 1 );
panel.revalidate(); // this invokes the layout manager
//panel.repaint(); // sometimes needed to force repainting of panel.