How can I find a JTextField under another JTextField?

In your attempts at trying GridBagLayout, assume that either you are not using GridBagConstraints when adding components, or that you are using it incorrectly.

If you want to center the text components in the GUI one on top of the other and tell them to put them in their own box, then use a GridBagLayout on the container that holds them and also pass in the appropriate GridBagConstraints that will work well for your desire. This would mean giving the constraints an appropriate gridx and gridy value to match where in the grid you want to place the component. Also, you will want to bind the component properly, and you will generally want to set constraint inserts to give an empty buffer of space around your components so they don't clump together. Myself, when I do something like this, I often use a 4 x 4 grid with two rows and two columns, including the JLabels column on the left so that the user knowswhich is each text component in the right column. I often use a helper method to create my constraints, something like this:

private GridBagConstraints createGbc(int x, int y) {
    GridBagConstraints gbc = new GridBagConstraints();
    gbc.gridx = x;
    gbc.gridy = y;
    gbc.fill = GridBagConstraints.HORIZONTAL; // stretch components horizontally
    gbc.weightx = 1.0;
    gbc.weighty = 0.0; // increase if you want component location to stretch vert.

    // I_GAP is a constant and is the size of the gap around
    // each component
    gbc.insets = new Insets(I_GAP, I_GAP, I_GAP, I_GAP);

    // if the x value is odd, anchor to the left, otherwise if even to the right
    gbc.anchor = x % 2 == 0 ? GridBagConstraints.WEST : GridBagConstraints.EAST;
    return gbc;
}

      

        
        
        
      

    

And then I will use it like this:

JPanel innerPanel = new JPanel(new GridBagLayout());

JLabel userNameLabel = new JLabel("User Name:");
userNameLabel.setForeground(Color.LIGHT_GRAY);
innerPanel.add(userNameLabel, createGbc(0, 0)); // add w/ GBC
innerPanel.add(tfUsername, createGbc(1, 0)); // etc...
JLabel passwordLabel = new JLabel("Password:");
passwordLabel.setForeground(Color.LIGHT_GRAY);
innerPanel.add(passwordLabel, createGbc(0, 1));
innerPanel.add(tfPassword, createGbc(1, 1));

      

        
        
        
      

    

A working example might look like this:

import java.awt.Color;
import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.GridBagConstraints;
import java.awt.GridBagLayout;
import java.awt.Insets;
import java.awt.Window;
import java.awt.Dialog.ModalityType;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.image.BufferedImage;
import java.io.IOException;
import java.net.URL;    
import javax.imageio.ImageIO;
import javax.swing.*;
import javax.swing.border.Border;

@SuppressWarnings("serial")
public class MetaSpaceLauncherPanel extends JPanel {
    // path to a public starry image
    public static final String IMG_PATH = "https://upload.wikimedia.org/wikipedia/"
            + "commons/thumb/b/be/Milky_Way_at_Concordia_Camp%2C_Karakoram_Range%2"
            + "C_Pakistan.jpg/1280px-Milky_Way_at_Concordia_Camp%2C_Karakoram_Range"
            + "%2C_Pakistan.jpg";
    private static final int I_GAP = 10;

    private static final int COLS = 15;
    private JTextField tfUsername = new JTextField(COLS);
    private JPasswordField tfPassword = new JPasswordField(COLS);
    private BufferedImage background = null;

    public MetaSpaceLauncherPanel(BufferedImage background) {
        this.background = background;

        // close window if enter pressed and data within fields
        ActionListener listener = e -> {
            String userName = tfUsername.getText().trim();
            char[] password = tfPassword.getPassword();

            Window window = SwingUtilities.getWindowAncestor(MetaSpaceLauncherPanel.this);
            if (userName.isEmpty() || password.length == 0) {
                // both fields need to be filled!
                String message = "Both user name and password fields must contain data";
                String title = "Invalid Data Entry";
                JOptionPane.showMessageDialog(window, message, title, JOptionPane.ERROR_MESSAGE);
            } else {
                // simply close the dialog
                window.dispose();
            }
        };

        tfUsername.addActionListener(listener);
        tfPassword.addActionListener(listener);

        JPanel innerPanel = new JPanel(new GridBagLayout());
        innerPanel.setOpaque(false);
        Border outerBorder = BorderFactory.createEtchedBorder();
        Border innerBorder = BorderFactory.createEmptyBorder(I_GAP, I_GAP, I_GAP, I_GAP);
        Border border = BorderFactory.createCompoundBorder(outerBorder, innerBorder);
        innerPanel.setBorder(border);

        JLabel userNameLabel = new JLabel("User Name:");
        userNameLabel.setForeground(Color.LIGHT_GRAY);
        innerPanel.add(userNameLabel, createGbc(0, 0));
        innerPanel.add(tfUsername, createGbc(1, 0));
        JLabel passwordLabel = new JLabel("Password:");
        passwordLabel.setForeground(Color.LIGHT_GRAY);
        innerPanel.add(passwordLabel, createGbc(0, 1));
        innerPanel.add(tfPassword, createGbc(1, 1));

        setLayout(new GridBagLayout());
        add(innerPanel); // add without constraints to center it
    }

    public String getUserName() {
        return tfUsername.getText();
    }

    public char[] getPassword() {
        return tfPassword.getPassword();
    }

    private GridBagConstraints createGbc(int x, int y) {
        GridBagConstraints gbc = new GridBagConstraints();
        gbc.gridx = x;
        gbc.gridy = y;
        gbc.fill = GridBagConstraints.HORIZONTAL; // stretch components horizontally
        gbc.weightx = 1.0;
        gbc.weighty = 0.0; // increase if you want component location to stretch vert.

        // I_GAP is a constant and is the size of the gap around
        // each component
        gbc.insets = new Insets(I_GAP, I_GAP, I_GAP, I_GAP);

        // if the x value is odd, anchor to the left, otherwise if even to the right
        gbc.anchor = x % 2 == 0 ? GridBagConstraints.WEST : GridBagConstraints.EAST;
        return gbc;
    }

    @Override
    protected void paintComponent(Graphics g) {
        super.paintComponent(g);
        if (background != null) {
            g.drawImage(background, 0, 0, this);
        }
    }

    @Override
    public Dimension getPreferredSize() {
        if (isPreferredSizeSet() || background == null) {
            return super.getPreferredSize();
        }
        int w = background.getWidth();
        int h = background.getHeight();
        return new Dimension(w, h);
    }

    private static void createAndShowGui() {
        BufferedImage img = null;
        try {
            // just using this as an example image, one available to all
            // you would probably use your own image
            URL imgUrl = new URL(IMG_PATH); // online path to starry image
            img = ImageIO.read(imgUrl);
        } catch (IOException e) {
            e.printStackTrace();
            System.exit(-1); // no image available -- exit!
        }
        MetaSpaceLauncherPanel launcherPanel = new MetaSpaceLauncherPanel(img);

        JDialog dialog = new JDialog(null, "MetaSpace Launcher", ModalityType.APPLICATION_MODAL);
        dialog.setDefaultCloseOperation(JDialog.DISPOSE_ON_CLOSE);
        dialog.getContentPane().add(launcherPanel);
        dialog.pack();
        dialog.setLocationRelativeTo(null);
        dialog.setVisible(true);

        // test to see if we can get the data
        String userName = launcherPanel.getUserName();
        char[] password = launcherPanel.getPassword();

        // don't convert password into String as I'm doing below as it is now
        // not secure
        String message = String.format("<html>User Name: %s<br/>Password: %s</html>", userName,
                new String(password));
        JOptionPane.showMessageDialog(null, message);
    }

    public static void main(String[] args) {
        SwingUtilities.invokeLater(() -> createAndShowGui());
    }
}