Show ScrollBars in FlowLayout only when necessary

Question

I will rephrase my question:**

How to prevent Java ScrollBar from being enabled in FlowLayout when there is enough space to show all items by warping them.

**

Here a screenshot of what I am trying to achieve:

Note that scrollbar is disabled when it is not necessary.

And when you resize the window scrollbar should appear if some items are out the viewplane

P.S. I am aware of things called documentation and web.

Answer 1

Your updated screen shots suggest that you may be looking for Wrap Layout, which "extends the FlowLayout" in a way that "will result in synchronizing the preferred size of the container with the layout of the container." See also Creating a Custom Layout Manager.

Answer 2

The default scroll bar policy is as needed, but I remember having to account for the FlowLayout gaps to get an even number initially. If you stretch this example out, you'll see the horizontal scroll bar disappear.

Update: It doesn't really fix your problem, but it shows how I implemented Scrollable. I wanted to get rid of setPreferredSize(), even tho' the image is just a placeholder. The Wrap Layout article talks about why FlowLayout works the way it does.

import javax.swing.*;
import java.awt.*;
import java.util.Random;

public class ImageScrollTest extends JPanel implements Scrollable {

    private static final int N = 8;
    private static final int W = 120;
    private static final int H = 100;
    private final FlowLayout layout = new FlowLayout();
    private final int hGap = layout.getHgap();
    private final int vGap = layout.getVgap();
    private final Dimension size;

    // Show n of N images in a Scrollable FlowLayout
    public ImageScrollTest(int n) {
        setLayout(layout);
        for (int i = 0; i < N; i++) {
            this.add(new ImagePanel());
        }
        size = new Dimension(n * W + (n + 1) * hGap, H + 2 * vGap);
    }

    @Override
    public Dimension getPreferredScrollableViewportSize() {
        return size;
    }

    @Override
    public int getScrollableUnitIncrement(
        Rectangle visibleRect, int orientation, int direction) {
        return getIncrement(orientation);
    }

    @Override
    public int getScrollableBlockIncrement(
        Rectangle visibleRect, int orientation, int direction) {
        return getIncrement(orientation);
    }

    private int getIncrement(int orientation) {
        if (orientation == JScrollBar.HORIZONTAL) {
            return W + hGap;
        } else {
            return H + vGap;
        }
    }

    @Override
    public boolean getScrollableTracksViewportWidth() {
        return false;
    }

    @Override
    public boolean getScrollableTracksViewportHeight() {
        return false;
    }

    private static class ImagePanel extends JPanel {

        private static final Random rnd = new Random();
        private Color color = new Color(rnd.nextInt());

        public ImagePanel() {
            this.setBackground(color);
            this.setBorder(BorderFactory.createLineBorder(Color.blue));
        }

        @Override
        public Dimension getPreferredSize() {
            return new Dimension(W, H);
        }
    }

    public static void main(String[] args) {
        EventQueue.invokeLater(new Runnable() {

            @Override
            public void run() {
                createAndShowGUI();
            }
        });
    }

    private static void createAndShowGUI() {
        JFrame f = new JFrame();
        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        ImageScrollTest ist = new ImageScrollTest(N / 2);
        JScrollPane sp = new JScrollPane(ist);
        sp.setVerticalScrollBarPolicy(JScrollPane.VERTICAL_SCROLLBAR_NEVER);
        f.add(sp);
        f.pack();
        f.setVisible(true);
    }
}