GridBagLayout : unexpected space between component

The issue is that the GridBagLayout doesn't know how to allocate a width to a column unless at least one component is added to the column with a "gridwidth = 1".

In this case the issue is between buttons 4 and 5:

1. button 4 is added to column 2 but has a gridwidth of 3, so column 2 effectively has the default width of 0.
2. button 5 attempts to span 3 columns but its preferred width is greater the the widths of the 3 columns.

Run you code removing button 4 from the layout:

//pane.add(button4, gbc4);

The layout is compressed.

Now look at button 5 with a width of 246.

This indicates the width of the first 3 columns should be at least 246.

Buttons 1 and 2 only have widths of 46 and 118 for a total of 164, which implies that column 2 should have a width of 82 (246 - 164).

However, the GridBagLayout is using 0, for the width of the column. This somehow is then adding the extra 82 pixels of space after button 4 leaving the gap between it and button 8.

The solution is to give column2 a minimum width. So even if no component with a gridwidth =1 is added to the column, a non-zero default width can be used in the calculations.

This is done with the following code:

    //pane.setLayout(new GridBagLayout());
    GridBagLayout gbl = new GridBagLayout();
    pane.setLayout(gbl);

    //  Set up a layout with 5 columns.
    //  minimimum width of a column 2 is 82 pixels

    int[] columns = new int[5];
    columns[2] = 82;
    gbl.columnWidths = columns;

This is not really a solution because it is to manual to attempt to calculate the minimum width.

It is only an attempt at an explanation of what is happening. The concept of applying minimum values to columns can be useful in other situations. See: Creating a board game layout using JLayeredPane for a working example of this concept.

Note, when you use the "fill" constraint as suggested by VGR then buttons 4, 6, 7 will be increased in size by an extra 82 pixels. So this addresses the visual gap by increasing the preferred widths of the buttons affected.

Without knowing the actual requirement and why such weird preferred sizes are being used we can't really give a better solution.

Edit:

the size of the buttons (height, width) is used to represent somes values, so it's important to maintain as they are

Then you may need so use nested panel with different layout managers to achieve you desired layout.

For example you might have a parent panel with a BorderLayout. Then you use create a panel with buttons 1-7 using your existing code and add this panel to the BorderLayout.CENTER. Then you add button 8 to the BorderLayout.LINE_END.

Don’t use setPreferredSize. You are interfering with the layout’s ability to place the components properly.

What you want is to set the fill constraint to BOTH. For instance:

gbc1.fill = GridBagConstraints.BOTH;

This will cause button1 to fill up its cell. Repeat this for each constraints object.

If you want a button to be even larger, you can change its margin, or you can use the ipadx and ipady constraints, which add to a component’s default size in the layout.

Side note: Your use of setPreferredSize prevents me from seeing the "8" in the rightmost button. Fonts are rendered differently on different computers. This is why you normally want to let a component report its own preferred size.

Here's the GUI I came up with.

I had to create two separate JPanels. One JPanel for the 1 - 4 JButtons and one JPanel for the 5 - 7 JButtons.

I had to set a preferred size for every JButton. I made a 1 x 1 button 60 x 40 pixels.

Here's the complete runnable code.

import java.awt.BorderLayout;
import java.awt.Dimension;
import java.awt.GridBagConstraints;
import java.awt.GridBagLayout;

import javax.swing.BorderFactory;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JPanel;

public class GridBagLayout8Button {
    
    public static void main(String[] args) {
        GridBagLayout8Button gbb = new GridBagLayout8Button();
        javax.swing.SwingUtilities.invokeLater(new Runnable() {
            public void run() {
                JFrame frame = new JFrame("GridBagLayout Test");
                frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                frame.add(gbb.createMainPanel(), BorderLayout.CENTER);
                frame.pack();
                frame.setLocationByPlatform(true);
                frame.setVisible(true);
            }
        });
    }
    
    public JPanel createMainPanel() {
        JPanel panel = new JPanel(new GridBagLayout());
        panel.setBorder(BorderFactory.createEmptyBorder(10, 10, 10, 10));
        
        addPanel(panel, createUpperPanel(), 0, 0, 1, 1);
        addPanel(panel, createLowerPanel(), 0, 1, 1, 1);
        createJButton(panel, "8", 1, 0, 1, 2);
        
        return panel;
    }
    
    private void addPanel(JPanel panel, JPanel innerPanel, int gridx, int gridy,
            int gridwidth, int gridheight) {
         GridBagConstraints gbc = new GridBagConstraints();
         gbc.anchor = GridBagConstraints.LINE_START;
         gbc.fill = GridBagConstraints.BOTH;
         gbc.gridx = gridx;
         gbc.gridy = gridy;
         gbc.gridwidth = gridwidth;
         gbc.gridheight = gridheight;
         
         panel.add(innerPanel, gbc);
    }
    
    private JPanel createUpperPanel() {
        JPanel panel = new JPanel(new GridBagLayout());
        
        createJButton(panel, "1", 0, 0, 1, 2);
        createJButton(panel, "2", 1, 0, 2, 1);
        createJButton(panel, "3", 1, 1, 2, 1); 
        createJButton(panel, "4", 3, 0, 3, 2);
        
        return panel;
    }
    
    private JPanel createLowerPanel() {
        JPanel panel = new JPanel(new GridBagLayout());
        
        createJButton(panel, "5", 0, 2, 4, 2);
        createJButton(panel, "6", 4, 2, 2, 1);
        createJButton(panel, "7", 4, 3, 2, 1);
        
        return panel;
    }

    private void createJButton(JPanel panel, String value, int gridx, int gridy,
            int gridwidth, int gridheight) {
        JButton button = new JButton(value);
        Dimension d = new Dimension(gridwidth * 60, gridheight * 40);
        button.setPreferredSize(d);
 
        GridBagConstraints gbc = new GridBagConstraints();
        gbc.anchor = GridBagConstraints.LINE_START;
        gbc.fill = GridBagConstraints.BOTH;
        gbc.gridx = gridx;
        gbc.gridy = gridy;
        gbc.gridwidth = gridwidth;
        gbc.gridheight = gridheight;
        
        panel.add(button, gbc);
    }

}