import javax.swing.*;
import java.awt.*;

public class HorizontalLine extends JPanel {
    @Override
    protected void paintComponent(Graphics g) {
        super.paintComponent(g);
        g.drawLine(0, getHeight() / 2, getWidth(), getHeight() / 2);
    }

    public static void main(String[] args) {
        JFrame frame = new JFrame("Horizontal Line");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setSize(400, 300);
        frame.add(new HorizontalLine());
        frame.setVisible(true);
    }
}