Java GridBagLayout - How to position my components gap-less and one by one?

Question

I'm using GridBagLayout to place my GUI components by the following code, wanting the components lay one by one in a column, without any gaps :

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

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

public class TestGUI extends JFrame{

    public TestGUI(){

        JPanel bigPanel = new JPanel(new GridBagLayout());
        JPanel panel_a = new JPanel();
        JButton btnA = new JButton("button a");
        panel_a.add(btnA);

        JPanel panel_b = new JPanel();
        JButton btnB = new JButton("button b");
        panel_b.add(btnB);

        GridBagConstraints c = new GridBagConstraints();
        c.gridx = 0;
        c.gridy = 0;
        c.weighty = 1D;
        c.fill = GridBagConstraints.HORIZONTAL;
        c.anchor = GridBagConstraints.NORTH;
        bigPanel.add(panel_a, c);

        c.gridx = 0;
        c.gridy = 1;
        c.fill = GridBagConstraints.HORIZONTAL;
        bigPanel.add(panel_b, c);

        this.add(bigPanel);
    }

    public static void main(String[] args) {

        TestGUI gui = new TestGUI();
        gui.setVisible(true);
        gui.pack();
    }
}

I wish the panels will be shown one by one in the column. but now i got this :

As i am going to add some more components in the bigPanel, and required some more customization to the layout, so i need to use GridBagLayout instead of other Layout.

Answer 1

You need to add an extra component so that it will fill the rest of the available space and push the two button-panels to the top. When you will add more components, you can of course remove that component.

Another option (without requiring an extra component) would have been to set weighty=1.0 for the panel_b and anchor=NORTH, but then you would have to change that when you add more components.

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

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

public class TestGUI extends JFrame {

    public TestGUI() {

        JPanel bigPanel = new JPanel(new GridBagLayout());
        JPanel panel_a = new JPanel();
        JButton btnA = new JButton("button a");
        panel_a.add(btnA);

        JPanel panel_b = new JPanel();
        JButton btnB = new JButton("button b");
        panel_b.add(btnB);

        GridBagConstraints c = new GridBagConstraints();
        c.gridwidth = GridBagConstraints.REMAINDER;
        c.fill = GridBagConstraints.HORIZONTAL;
        c.weightx = 1.0;
        bigPanel.add(panel_a, c);
        bigPanel.add(panel_b, c);
        c.weighty = 1.0;
        // Temporary panel to fill the rest of the bigPanel
        bigPanel.add(new JPanel(), c);
        this.add(bigPanel);
    }

    public static void main(String[] args) {
        SwingUtilities.invokeLater(new Runnable() {
            @Override
            public void run() {
                TestGUI gui = new TestGUI();
                gui.pack();
                gui.setVisible(true);
            }
        });
    }
}