Problem med grafik i Java

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

public class main {

	public static void main(String[] args) {
		JFrame frame = new JFrame();
		frame.setSize(800, 600);
		frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
		frame.setBackground(Color.BLUE);
		frame.setVisible(true);
		Graphics g = frame.getGraphics();
		g.setColor(Color.BLACK);
		g.fillRect(100, 100, 50, 50);
		frame.repaint();
	}

}

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

public class main {

	public static void main(String[] args) {
		JFrame frame = new JFrame();
		MyJPanel panel = new MyJPanel();
		frame.setSize(800, 600);
		frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
		frame.setVisible(true);
		frame.add(panel);
	}

}

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

public class MyJPanel extends JPanel {
	
	MyJPanel(){
		this.setBackground(Color.BLUE);
	}

	public void paintComponent(Graphics g){
		g.setColor(Color.BLACK);
		g.fillRect(100, 100, 50, 50);
	}	
}