Too many JPanels inside a JPanel (with GridBagLayout)

Question

So basically if I put JPanels inside a JPanel that uses GridBagLayout and I restrict the size with setPreferredSize, even

Answer 1

The end goal is to remove older panels when a new panel is added and there isn't enough room for it

I would guess that after you add a panel you compare the preferred height with the actual height. When the preferred height is greater you have a problem and you remove components as required.

So then the next problem is to use a layout manager that doesn't change the heights of the panels. This can still be done with the GridBagLayout. You just need to override the getMinimumSize() method to return the getPreferredSize() Dimension.

Each part of the accordion can open individually and they're of arbitrary size and get added on the fly

You might want to consider using the Relative Layout. You can add components whose preferred size will be respected. So you will be able to check when the preferred height is greater than the actual height.

Then you can also add components that will be sized based on the amount of space left in the panel. These would be your expanding panels.

So in your example you example when you expand an item you could configure that component to take up the entire space available. If you expand two items then they would each get half the space available.

Maybe something like this:

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;

public class ExpandingPanel extends JPanel
{
    private JPanel expanding;

    public ExpandingPanel(String text, Color color)
    {
        setLayout( new BorderLayout() );

        JButton button = new JButton( text );
        add(button, BorderLayout.NORTH);

        expanding = new JPanel();
        expanding.setBackground( color );
        expanding.setVisible( false );
        add(expanding, BorderLayout.CENTER);

        button.addActionListener( new ActionListener()
        {
            @Override
            public void actionPerformed(ActionEvent e)
            {
                expanding.setVisible( !expanding.isVisible() );

                Container parent = ExpandingPanel.this.getParent();
                LayoutManager2 layout = (LayoutManager2)parent.getLayout();

                if (expanding.isVisible())
                    layout.addLayoutComponent(ExpandingPanel.this, new Float(1));
                else
                    layout.addLayoutComponent(ExpandingPanel.this, null);

                revalidate();
                repaint();
            }
        });
    }

    private static void createAndShowGUI()
    {
        RelativeLayout rl = new RelativeLayout(RelativeLayout.Y_AXIS);
        rl.setFill( true );

        JPanel content = new JPanel( rl );
        content.add( new ExpandingPanel("Red", Color.RED) );
        content.add( new ExpandingPanel("Blue", Color.BLUE) );
        content.add( new ExpandingPanel("Green", Color.GREEN) );

        JFrame frame = new JFrame("Expanding Panel");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.add( content);
        frame.setLocationByPlatform( true );
        frame.setSize(200, 300);
        frame.setVisible( true );
    }

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