Thread: Timed matrix generation

Hello

I've been playing around with 2D graphics in Java and I want to make a timed matrix generator using Thread.sleep() .
Simply put , I have a matrix of squares which is 20x20 blue squares and I want to fill each square in order at 10ms intervals.
I tried doing that using the following code but the repaint() doesn't work for some reason. It just waits until the whole matrix is updated and then it repaints .
Can someone tell me why it's not working?

You can see in the method "Generator" it updates the specific square with a boolean value , sleeps for 10 ms and then performs the repaint();
But for some reason the repaint() is not called until the whole thing is done.

Thanks in advance

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
 
public class GUI extends JPanel implements MouseListener, MouseMotionListener {
 
   boolean[][] Matrix;
 
    public static void main(String[] args) throws InterruptedException {
 
        GUI gui = new GUI();
        JFrame f  =  new JFrame("graphix"); 
        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); // Finishes program when Cross is pressed on window
        f.setSize(815,835);       // Size of main window 
        f.setVisible(true);       // Has to be set true to be visible
        f.add(gui);                // Adds instance of the user interface class
 
    }
 
 
    public GUI() throws InterruptedException
    {
        Matrix = new boolean[20][20];
        generator();
    }
 
    public  void generator() throws InterruptedException{
 
        for(int x=0; x < 20; x ++){
            for(int y=0; y< 20; y++){
                Matrix[x][y] = true;
                Thread.sleep(10);
                repaint();
            }
        }
 
 
    }
 
    @Override
    public void paintComponent(Graphics g) {
 
        super.paintComponent(g);          
        this.setBackground(Color.BLACK);   // Window Background
 
        g.setColor(new Color(0, 0, 255));
 
 
        for (int x = 0; x < 20; x++) {
            for (int y = 0; y < 20; y++) {
                if (Matrix[x][y]) {
                    g.fillRect(x * 40, y * 40, 38, 38);
                }
            }    // end of for y    
        } // end of for x
 
    }

I removed the generator() function and wrote "GUI.generator()" to after the declaration of the JFrame and it seems to be working as I intended now, thanks!
I do not know about the swing timer ,multithreading or the EDT control and i'm planning to study all that in the holidays before my second semester starts .
I just wanted to get this running to play with diffrent patterns while I study those topics so thanks for the help.

Also I found instructions on how to make nanosecond delays using:

long start = System.nanoTime();
             while(start + 10000 >= System.nanoTime());

Pretty fun stuff