Buttons not visible until mouseover

I created a frame and panel (for Graphics) and add buttons to the panel. However, when I run my program buttons are not visible until I hover over them. Maybe it is something wrong with graphics methods.

How the problem can be solved?

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

public class Main{
    public static void main(String[] args) {
        
        JFrame f = new JFrame();
        f.setSize(600,500);
        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        f.setResizable(false);
        
        Panel panel = new Panel();
        
        f.add(panel);
        f.setVisible(true);

    }
}

class Panel extends JPanel implements ActionListener{
    int[] x = {200,400,300,200};
    int[] y = {100,100,200,100};
    JButton fillRed;
    JButton fillBlack;
    JButton cancel;
    Panel(){
         
        this.setLayout(null);
        fillRed = new JButton("Red");
        fillRed.setBounds(50, 400, 100, 50);
        fillRed.addActionListener(this);
        this.add(fillRed);
         
        fillBlack = new JButton("Black");
        fillBlack.setBounds(250, 400, 100, 50);
        fillBlack.addActionListener(this);
        this.add(fillBlack);
         
        cancel = new JButton("Cancel");
        cancel.setBounds(450,400,100,50);
        cancel.addActionListener(this);
        this.add(cancel);
         
    }
     
    public void paint(Graphics g) {
        super.paintComponent(g);
        g.drawPolygon(x,y,4);
        
    }
    public void fillRed(Graphics g) {
        super.paintComponent(g);
        g.setColor(Color.RED);
        g.fillPolygon(x,y,4);
        
    }
    public void fillBlack(Graphics g) {
        super.paintComponent(g);
        g.setColor(Color.BLACK);
        g.fillPolygon(x,y,4);
        
    }
    public void cancel(Graphics g) {    
        super.paintComponent(g);
        g.drawPolygon(x,y,4);
        repaint();
    }

    @Override
    public void actionPerformed(ActionEvent e) {
        
        if(e.getSource()==fillRed) {
             fillRed(getGraphics());
        }
        
        if(e.getSource()==fillBlack) {
             fillBlack(getGraphics());
        }
        
        if(e.getSource()==cancel) {
             cancel(getGraphics());
        }
    }
}

Answer

Your painting code is mostly wrong. For example:

public void paint(Graphics g) {
    super.paintComponent(g);
    g.drawPolygon(x,y,4);
    
}

If you need to override paint() then the first statement should be super.paint(g).

However, you should NOT override paint. For custom painting you override paintComponent(...) and then invoke super.paintComponent(g). Read the Swing tutorial on Custom Painting for more information and working examples.

public void fillBlack(Graphics g) {
    super.paintComponent(g);
    g.setColor(Color.BLACK);
    g.fillPolygon(x,y,4);
}

Never invoke paintComponent(…) directly. If you need to change a property of a component then you invoke repaint() and Swing will invoke the painting methods.

    if(e.getSource()==fillRed) {
         fillRed(getGraphics());
    }

Don’t invoke getGraphics() on a Swing component. Painting is done by setting properties of your class and then you invoke repaint(). If you need to paint multiple objects then you need to keep track of each object you want to paint and then in the painting method you paint every object. Check out Custom Painting Approaches for example of how this can be done.