Swing example program

import java.awt.Color;
import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.GridBagLayout;
import java.awt.GridLayout;
import javax.swing.*;

@SuppressWarnings("serial")
public class Gui002 extends JPanel {
    private static final int TEXT_FIELD_COUNT = 3;
    private static final Color BG = Color.BLACK;
    private static final Dimension PREF_SIZE = new Dimension(500, 350);
    private JTextField[] fields = new JTextField[TEXT_FIELD_COUNT];

    public Gui002() {
        // make background black
        setBackground(BG);

        // create JPanel to hold 3 JTextFields in a single column
        JPanel innerPanel = new JPanel(new GridLayout(0, 1, 10, 0));
        // allow background color to show through this JPanel
        innerPanel.setOpaque(false);

        // create and add our 3 JTextFields
        for (int i = 0; i < fields.length; i++) {
            fields[i] = new JTextField(10);
            innerPanel.add(fields[i]);
        }

        setPreferredSize(PREF_SIZE); // not best way to do this
        // set layout of enclosing JPanel so that inner panel is centered
        setLayout(new GridBagLayout());
        add(innerPanel);
    }

    @Override
    protected void paintComponent(Graphics g) {
        super.paintComponent(g);

        // .... only painting code goes here
    }

    private static void createAndShowGui() {
        Gui002 mainPanel = new Gui002();

        JFrame frame = new JFrame("GUI");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.getContentPane().add(mainPanel);  // add components first
        frame.pack();
        frame.setLocationRelativeTo(null);
        frame.setVisible(true);  // set visible **after** components added
    }

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