Java Swing - JPanel centering issue

Question

Basically, this is the structure of my Window.

The actual window looks like this:

My problem is that I want the MainPanel with the ComboBox and and the rest to fit the JFrame window. Stretch horizontally and stick to the top. But I've tried all kinds of layouts (Box, GridBag, Grid). It just remains centered. How do I achieve this?

Here is the MVCC:

    import java.awt.CardLayout;
    import java.awt.FlowLayout;
    import java.awt.GridBagConstraints;
    import java.awt.GridBagLayout;

    import javax.swing.*;
    import javax.swing.UIManager.LookAndFeelInfo;





    public class simplelayout{
    JFrame mainFrame;
    JPanel mainPanel;
    JPanel innerPanel;
    JPanel createDomainPanel;
    JPanel listDomainPanel = new JPanel(new GridBagLayout());
    String comboBoxItems[]={"Create Domain","List Domains"};
    @SuppressWarnings("rawtypes")
    JComboBox listCombo;
    JButton goBtn;
    CardLayout layout;
    JTextField domainName = new JTextField(); 
    JLabel successMessage = new JLabel("");


    public simplelayout(){
        initialize();
        setCardLayout();
    }

    @SuppressWarnings({ "rawtypes", "unchecked" })
    private void setCardLayout() {
        innerPanel = new JPanel();
        //innerPanel.setSize(600, 400);

        layout = new CardLayout(10,10);
        innerPanel.setLayout(layout);

        //Default Panel
        JPanel defaultPanel = new JPanel(new FlowLayout());
        defaultPanel.add(new JLabel("Welcome to Amazon Simple DB"));

        //Create Domain Panel
        createDomainPanel = new JPanel(new GridBagLayout());
        GridBagConstraints gbc = new GridBagConstraints();
        //createDomainPanel.setSize(600, 300);
        gbc.fill=GridBagConstraints.HORIZONTAL;


        gbc.gridx=0;
        gbc.gridy=0;
        createDomainPanel.add(new JLabel("Enter the name of the domain"), gbc);

        gbc.gridx=0;
        gbc.gridy=1;        
        createDomainPanel.add(domainName, gbc);

        JPanel result = new JPanel(new FlowLayout());
        result.add(successMessage);
        gbc.anchor=GridBagConstraints.LAST_LINE_START;
        gbc.gridx=0;
        gbc.gridy=2;
        createDomainPanel.add(result, gbc);

        //List Domain Panel
        listDomainPanel.add(new JLabel("List of Domains"), gbc);

        //Add to innerPanel
        innerPanel.add(defaultPanel, "Default");
        innerPanel.add(createDomainPanel, "Create Domain");
        innerPanel.add(listDomainPanel, "List Domain");

        layout.show(innerPanel, "Default");

        DefaultComboBoxModel panelName = new DefaultComboBoxModel();
        panelName.addElement("Create Domain");
        panelName.addElement("List Domain");

        listCombo = new JComboBox(panelName);
        listCombo.setSelectedIndex(0);
        JScrollPane listComboScroll = new JScrollPane(listCombo);

        goBtn = new JButton("Go");



        mainPanel.setLayout(new GridBagLayout());
        GridBagConstraints gbc1 = new GridBagConstraints();
        gbc1.fill=GridBagConstraints.HORIZONTAL;
        gbc1.anchor=GridBagConstraints.FIRST_LINE_START;

        gbc1.gridx = 0;
        gbc1.gridx = 0;
        mainPanel.add(listComboScroll, gbc1);

        gbc1.gridy = 1;
        mainPanel.add(goBtn, gbc1);

        gbc1.gridy = 2;
        mainPanel.add(innerPanel, gbc1);

        GridBagConstraints gbc2 = new GridBagConstraints();
        gbc2.fill=GridBagConstraints.HORIZONTAL;
        gbc2.anchor=GridBagConstraints.FIRST_LINE_START;
        JScrollPane scr = new JScrollPane(mainPanel);
        mainPanel.setSize(mainFrame.getPreferredSize());
        mainFrame.add(scr, gbc);
        mainFrame.setVisible(true);
        mainFrame.setExtendedState(JFrame.MAXIMIZED_BOTH);
    }




    private void initialize() {
        mainFrame = new JFrame("Amazon Simple DB");
        mainFrame.setSize(700, 500);
        mainFrame.setLayout(new GridBagLayout());
        mainFrame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        mainPanel = new JPanel();
        //mainPanel.setLayout(new GridLayout(8,1));

        mainFrame.setVisible(true);


    }

    public static void main(String[] args) {

        try {

            //UIManager.put("nimbusBase", new Color(178,56,249));
            //UIManager.put("nimbusBlueGrey", new Color(102,212,199));
            //UIManager.put("control", new Color(212,133,102));

            for (LookAndFeelInfo info : UIManager.getInstalledLookAndFeels()) {
                if ("Nimbus".equals(info.getName())) {
                    UIManager.setLookAndFeel(info.getClassName());
                    break;
                }
            }
        } catch (Exception e) {
            // If Nimbus is not available, you can set the GUI to another look and feel.
        }

        SwingUtilities.invokeLater(new Runnable() {

            @Override
            public void run() {
                new simplelayout();

            }
        });

    }
    }

Answer 1

It sounds like you want to use the application frame's default BorderLayout, placing the combobox & button components in a nested panel in the BorderLayout.PAGE_START location with the innerPanel in the CENTER position.

Answer 2

Try to add a mainFrame.pack(); in the initialize() and the setCardLayout()methods, Remove all mainFrame.setSize() and mainPanel.setSize() methods.