Understanding the behaviour of GridBagLayout

Question

Right now I don't understand, why GridBagLayout from Java does what it does.

What I expected:

What I got:

My code:

import java.awt.Color;
import java.awt.GridBagConstraints;
import java.awt.GridBagLayout;

import javax.swing.JComponent;
import javax.swing.JFrame;
import javax.swing.JPanel;

public class LayoutTest extends JFrame {
    private static final long serialVersionUID = 1L;

    public static void main(String[] args) {
        LayoutTest t = new LayoutTest();
        t.setSize(640, 480);
        t.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        t.setVisible(true);
    }

    public LayoutTest() {
        JPanel content = new JPanel(new GridBagLayout());

        addToPanel(content, createTestPanel(Color.CYAN), 0, 0, 1, 1, 0.5d, 1.0d);
        addToPanel(content, createTestPanel(Color.BLUE), 1, 0, 2, 1, 1.0d, 1.0d);
        addToPanel(content, createTestPanel(Color.CYAN), 3, 0, 1, 1, 0.5d, 1.0d);
        addToPanel(content, createTestPanel(Color.YELLOW), 0, 1, 2, 1, 1.0d, 1.0d);
        addToPanel(content, createTestPanel(Color.GRAY), 2, 1, 2, 1, 1.0d, 1.0d);

        this.setContentPane(content);
    }

    private JPanel createTestPanel(Color c) {
        JPanel ret = new JPanel();
        
        ret.setBackground(c);
        
        return ret;
    }
    
    private void addToPanel(JPanel target, JComponent component, int x, int y, int width, int height, double weightx, double weighty) {
        GridBagConstraints gbc = new GridBagConstraints();
        
        gbc.anchor = GridBagConstraints.CENTER;
        gbc.fill = GridBagConstraints.BOTH;
        gbc.gridx = x;
        gbc.gridwidth = width;
        gbc.weightx = weightx;
        gbc.gridy = y;
        gbc.gridheight = height;
        gbc.weighty = weighty;
        
        target.add(component, gbc);
    }
}

Though I expected this code to create a layout where the blue rectangle has the same width as the yellow or the grey one whilst being centered in the upper row I don't seem to be able to make it work.

So why doesn't this work? What did I miss?

You can't just give a component a "gridwidth of 2". Check out: https://stackoverflow.com/a/57463596/131872 which attempts to explain why this doesn't work. — camickr, Dec 21 '22 at 02:46

hunter · Answer 1 · 2022-12-21T10:28:48.307

The way you specify the weightx , doesnt work as you expected, because when we specify a weightx for a cell which has gridwidth more than one, GridBagLayout doesnt know which column should be given the weightx.

in your case, layout has 4 columns, and all columns are equally weighted(according to your expected view) ,

so , don't set the weightx when component is added , so remove this code line

gbc.weightx = weightx;

then when GridBagLayout is created , set weightx for the layout , not when adding the components,

GridBagLayout layoutManager = new GridBagLayout();
layoutManager.columnWeights = new double[]{0.25,0.25,0.25,0.25};
JPanel content = new JPanel(layoutManager);

Understanding the behaviour of GridBagLayout

1 Answers1