Swing : GridBagLayout adding components not the expected way

Question

I'm just trying to use the GridBagLayout following this tutorial.

I can't get what I want to have.

I would want each Panel to take as much space as possible but it is not the case, I really tried everything, following the steps in the Oracle Docs

But the result is always the same :

I really do not understand why the layout is not working.

I tried changing the layout of the JFrame, setting the panel as ContentPane(), tried to add the constraints found on both websites, nothing seems to work.

Your help will is appreciated.

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Full Code

public class Monitor extends JFrame{

    // Menu
    private JMenuBar bar = new JMenuBar();
    private JMenu file = new JMenu("File");
    private JMenuItem open = new JMenuItem("Open");
    private JMenuItem exit = new JMenuItem("Exit");

    private JPanel mainPanel = new JPanel();

    private JPanel secondaryPanel;
    private JPanel explorerPanel, tagPanel;

    public Monitor(){
        setTitle("");
        setSize(750,600);
        setDefaultCloseOperation(EXIT_ON_CLOSE);
        setLocationRelativeTo(null);
        setResizable(false);


        initComps();
    }

    public void initComps(){

        explorerPanel = new JPanel();
        explorerPanel.setBackground(Color.BLACK);
        explorerPanel.setPreferredSize(new Dimension(250,300));

        tagPanel = new JPanel();
        tagPanel.setBackground(Color.yellow);
        tagPanel.setPreferredSize(new Dimension(250, 300));

        secondaryPanel = new JPanel();
        secondaryPanel.setBackground(Color.blue);
        secondaryPanel.setPreferredSize(new Dimension(500, 600));

        mainPanel = new JPanel();
        mainPanel.setPreferredSize(new Dimension(750, 600));
        mainPanel.setLayout(new GridBagLayout());

        GridBagConstraints gbc = new GridBagConstraints();

        gbc.gridx = 0 ; gbc.gridy = 0 ; gbc.gridwidth = 1 ; gbc.gridheight = 1;
        mainPanel.add(explorerPanel, gbc);

        gbc.gridx = 1 ; gbc.gridy = 0 ; gbc.gridwidth = 2 ; gbc.gridheight = 2;
        mainPanel.add(secondaryPanel, gbc);

        gbc.gridx = 0 ; gbc.gridy = 1 ; gbc.gridwidth = 1 ; gbc.gridheight = 1;
        mainPanel.add(tagPanel, gbc);

        this.setContentPane(mainPanel);

        // Menu Bar
        file.add(open);

        exit.addActionListener(new ActionListener(){
            @Override
            public void actionPerformed(ActionEvent e) {
                System.exit(0);
            }
        });
        exit.setMnemonic('q');
        file.add(exit);

        bar.add(file);
        setJMenuBar(bar);
    }

    public static void main(String[] args) {
        new Monitor().setVisible(true);

    }
}

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EDIT :

When adding

gbc.weightx = 1;
gbc.weighty = 1;

Result

Answer 1

Your problem comes from your dimensions.

Don't use "preferred" only. Use minimum as well.

explorerPanel = new JPanel();
explorerPanel.setBackground(Color.BLACK);
explorerPanel.setMinimumSize(new Dimension(250, 300));
explorerPanel.setPreferredSize(new Dimension(250, 300));

tagPanel = new JPanel();
tagPanel.setBackground(Color.yellow);
tagPanel.setMinimumSize(new Dimension(250, 300));
tagPanel.setPreferredSize(new Dimension(250, 300));

secondaryPanel = new JPanel();
secondaryPanel.setBackground(Color.blue);
secondaryPanel.setMinimumSize(new Dimension(500, 600));
secondaryPanel.setPreferredSize(new Dimension(500, 600));

mainPanel = new JPanel();
mainPanel.setPreferredSize(new Dimension(750, 600));
mainPanel.setMinimumSize(new Dimension(750, 600));
mainPanel.setLayout(new GridBagLayout());

This worked in my test: