JFrame graphics ignores the first few renders

Here is a minimal code to see the bug:

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

public class Main1 extends JFrame {
    static Main1 main;
    public Main1() {
        super("app");
    }

    public static void main(String[] args) {
        main = new Main1();
        main.setBounds(300, 300, 800, 500);
        main.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        main.setVisible(true);
        Graphics g = main.getGraphics();
        for(int i = 0; i < 100; i++){
            g.setColor(new Color(255, 0, 0));
            g.fillRect(0, 0, 800, 500);
        }
    }
}

If i use 100 in the "for" cycle, the frame appears not to be colored, but 200 loops is enough to color it.

I want to make an application where frames change rarely, but this feature ruins the quality of code because I have to make a number of dummy frames.

public static void main(String[] args) {
    main = new Main1();
    main.setBounds(300, 300, 800, 500);
    main.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    main.setVisible(true);
    Graphics g = main.getGraphics();
    for(int i = 0; i < 100; i++){
        g.setColor(new Color(255, 0, 0));
        g.fillRect(0, 0, 800, 500);
    }
}

This is not how you do Swing graphics. Getting a Graphics object by calling .getGraphics() on a component gives you a short-lived unstable and sometimes null object. For instance, it takes some time for the created JFrame to render, and if you call getGraphics() and try to use it prior to rendering, the object may be null, and certainly won't wokr.

Instead paint within a JPanel's paintComponent method using the Graphics object given by the JVM as per the tutorials:

public class MainPanel extends JPanel {

    public MainPanel {
        setPreferredSize(new Dimension(800, 500)));
        setBackground(new Color(255, 0, 0)); // if you just want to set background
    }

    protected void paintComponent(Graphics g) {
        super.paintComponent(g);
        
        // use g here do do your drawing
    }

}

and then use it like so:

public static void main(String[] args) {
    SwingUtilities.invokeLater(() -> {
        JFrame frame = new JFrame("GUI");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.add(new MainPanel());
        frame.pack();
        frame.setLocationRelativeTo(null);
        frame.setVisible(true);
    });
}  

Tutorial: Lesson: Performing Custom Painting

And yes, if you want to drive a simple animation, use a Swing Timer to help drive it like so:

public class MainPanel extends JPanel {
    private int x = 0;
    private int y = 0;

    public MainPanel {
        setPreferredSize(new Dimension(800, 500)));
        setBackground(new Color(255, 0, 0)); // if you just want to set background
        
        // timer code:
        int timerDelay = 15;
        new Timer(timerDelay, ()-> {
            x += 4;
            y += 4;
            repaint();
        }).start();
    }

    protected void paintComponent(Graphics g) {
        super.paintComponent(g);
        
        // use g here do do your drawing
        g.setColor(Color.BLUE);
        g.drawRect(x, y, 20, 20);
    }

}