component size

[Kevin Tysen]

I have in my program two JLists, which I put into two separate JScrollPanes, and I gave the two JScrollPanes the same preferred size.
The first JScrollPane is in a JPanel with horizontal BoxLayout, which is the default layout for JPanels. The JPanel is in the NORTH area of the JFrame. As far as I can tell, it has its preferred size.
The second JScrollPane is not on the JFrame together with the first JScrollPane. The second one is in a JPanel with a vertical BoxLayout. I tried putting that JPanel in different places. When it's NORTH or SOUTH, it gets its preferred height, but not its preferred width. When it's EAST or WEST, it gets its preferred width, but not its preferred height.
Why can't I get the second JScrollPane to its preferred height and width, like I can with the first JScrollPane?

[Craig Wood]

When it's NORTH or SOUTH, it gets its preferred height, but not its preferred width.
When it's EAST or WEST, it gets its preferred width, but not its preferred height.
As advertised: see the BorderLayout class api, specifically the paragraph in the comments
section that appears just above the demo app image.
The technique for getting these outer sections of BorderLayout to show your component at
its preferredSize is to add the component to another JPanel whose layout manager respects
the preferredSize of its child components. FlowLayout will do this. It also places the
component up toward the top and with an empty border. GridBagLayout is another layout
manager that respects the preferredSize of its children and, with the default constraints,
will show the component in the center with no empty border. In the test below note the
difference between the FlowLayout placement (blue and yellow) and the GridBagLayout
placements.
BoxLayout is a specialty layout and has additional subtleties that may require attention
to get it to behave the way you want. These include alignment and size peculiarities and
are outlined in the tutorial page How to Use BoxLayout. The test below attempts to get the
child component to be shown at its preferredSize in different configurations.

import java.awt.*;
import javax.swing.*;
 
public class Layout {
    JTabbedPane tabbedPane;
 
    private JTabbedPane getContent() {
        tabbedPane = new JTabbedPane();
        tabbedPane.addTab("Flow_GridBag", getFlowGridComponent());
        tabbedPane.addTab("BoxLayout",    getBoxComponent());
        return tabbedPane;
    }
 
    private JPanel getFlowGridComponent() {
        JPanel panel = new JPanel(new BorderLayout());
        panel.add(getFlowGridNorth(), "North");
        panel.add(getFlowGridWest(),  "West");
        panel.add(getFlowGridSouth(), "South");
        panel.add(getFlowGridEast(),  "East");
        return panel;
    }
 
    private JPanel getFlowGridNorth() {
        // Use a panel to simulate the JScrollPane whose
        // preferredSize has been set.
        JPanel child = getPanel(Color.blue);
        // To check the default layout manager of a component
        System.out.println("Default layout manager for JPanel = " +
                            child.getLayout().getClass().getName());
        JPanel panel = new JPanel();
        panel.setBorder(BorderFactory.createEtchedBorder());
        panel.add(child);
        // Does panel have a preferredSize?
        System.out.println("panel preferredSize = " +
                            panel.getPreferredSize());
        return panel;
    }
 
    private JPanel getFlowGridWest() {
        JPanel child = getPanel(Color.green.darker());
        JPanel panel = new JPanel(new GridBagLayout());
        GridBagConstraints gbc = new GridBagConstraints();
        panel.setBorder(BorderFactory.createEtchedBorder());
        panel.add(child, gbc);
        return panel;
    }
 
    private JPanel getFlowGridSouth() {
        JPanel child = getPanel(Color.red);
        JPanel panel = new JPanel(new GridBagLayout());
        GridBagConstraints gbc = new GridBagConstraints();
        panel.setBorder(BorderFactory.createEtchedBorder());
        panel.add(child, gbc);
        return panel;
    }
 
    private JPanel getFlowGridEast() {
        JPanel child = getPanel(Color.orange);
        JPanel panel = new JPanel();
        panel.setBorder(BorderFactory.createEtchedBorder());
        panel.add(child);
        return panel;
    }
 
    private JPanel getBoxComponent() {
        JPanel panel = new JPanel(new BorderLayout());
        panel.add(getBoxNorth(), "North");
        panel.add(getBoxWest(),  "West");
        panel.add(getBoxSouth(), "South");
        panel.add(getBoxEast(),  "East");
        return panel;
    }
 
    private JPanel getBoxNorth() {
        JPanel child = getPanel(Color.blue);
        child.setMaximumSize(child.getPreferredSize());
        JPanel panel = new JPanel();
        panel.setMaximumSize(child.getPreferredSize());
        BoxLayout box = new BoxLayout(panel, BoxLayout.X_AXIS);
        panel.setLayout(box);
        panel.setBorder(BorderFactory.createEtchedBorder());
        panel.add(Box.createHorizontalGlue());
        panel.add(child);
        panel.add(Box.createHorizontalGlue());
        return panel;
    }
 
    private JPanel getBoxWest() {
        JPanel child = getPanel(Color.green.darker());
        child.setMaximumSize(child.getPreferredSize());
        JPanel panel = new JPanel();
        BoxLayout box = new BoxLayout(panel, BoxLayout.X_AXIS);
        panel.setLayout(box);
        panel.setBorder(BorderFactory.createEtchedBorder());
        panel.add(child);
        return panel;
    }
 
    private JPanel getBoxSouth() {
        JPanel child = getPanel(Color.red);
        child.setMaximumSize(child.getPreferredSize());
        JPanel panel = new JPanel();
        BoxLayout box = new BoxLayout(panel, BoxLayout.Y_AXIS);
        panel.setLayout(box);
        panel.setBorder(BorderFactory.createEtchedBorder());
        panel.add(child);
        return panel;
    }
 
    private JPanel getBoxEast() {
        JPanel child = getPanel(Color.orange);
        child.setMaximumSize(child.getPreferredSize());
        JPanel panel = new JPanel();
        BoxLayout box = new BoxLayout(panel, BoxLayout.Y_AXIS);
        panel.setLayout(box);
        panel.setBorder(BorderFactory.createEtchedBorder());
        panel.add(Box.createVerticalGlue());
        panel.add(child);
        panel.add(Box.createVerticalGlue());
        return panel;
    }
 
    private JPanel getPanel(Color color) {
        JPanel panel = new JPanel();
        panel.setBackground(color);
        panel.setPreferredSize(new Dimension(100, 100));
        return panel;
    }
 
    public static void main(String[] args) {
        Layout test = new Layout();
        JFrame f = new JFrame();
        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        f.getContentPane().add(test.getContent());
        f.setSize(400,400);
        f.setLocation(200,200);
        f.setVisible(true);
    }
}

[Kevin Tysen]

Thank you. I had forgotten that the default layout for a JPanel is the FlowLayout. Now I know that FlowLayout respects preferred size, and Box Layout doesn't.