Java drawing weird visual artifacts / errors

Question

I am starting to program in Java. We were instructed to execute some algorithms. I got the Sierpinski Triangle. I had the idea to create a 2D array and store the values, 0 = white rectangle, 1 = blue rectangle. I had big problems to do this (never had experience with swings / awt). I finally did this, but a strange visual error appeared at the end of the drawing. This is not the end, but the lines continue.

I have no idea why this is.

Here is my code:

Oknowhich extends JPanel:

/*
 * To change this license header, choose License Headers in Project Properties.
 * To change this template file, choose Tools | Templates
 * and open the template in the editor.
 */
package newpackage;

import java.awt.Color;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.util.Arrays;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.SwingUtilities;

/**
 *
 * @author Juraj
 */
class Okno extends JPanel {

    // value = 1 is blue rectangle, value 0 = white rectangle
    public static int n = 500;
    public static int[][] tabulka = new int[n][n]; //creating 2D array

    public static void inicializaciaTabulky(){
        for (int i = 0; i < n; i++) { // initialization of 2D array (first column and first row = 1)
            tabulka[0][i] = 1;
            tabulka[i][0] = 1;
        }
    }

    // filling the rest of the array; if the cell above and cell to the left are the same value, e.g.
    // value of 1 or 0, then it is 0, everything else is 1
    public static void doplnenieTabulky() {
        for (int i = 1; i < n; i++) {
            for (int j = 1; j < n; j++) {
                if (tabulka[i-1][j] == 1 && tabulka[i][j-1] == 1 || 
                    tabulka[i-1][j] == 0 && tabulka[i][j-1] == 0) {
                    tabulka[i][j] = 0;
                } else {
                    tabulka[i][j] = 1;
                }
            }
        }
    }

    // drawing rectangles; if the value is 1 = blue rectangle, value 0 = white rectangle
    private void vykreslenie(Graphics g){
        Graphics2D g2d = (Graphics2D) g;

        for (int i = 0; i < n; i++) {
            for (int j = 0; j < n; j++) {
                switch(tabulka[i][j]) {
                    case 0: 
                        g.setColor(Color.white);
                        g.drawRect(i, j, 50, 50);
                        break;
                    case 1: 
                        g.setColor(Color.blue);
                        g.drawRect(i, j, 50, 50);
                        break;
                }
            }
        }
    }

    @Override
    public void paintComponent(Graphics g) {
        super.paintComponent(g);
        vykreslenie(g);
    }
}

Trojuholnikwhich extends JFrame:

public class Trojuholnik extends JFrame {  
    public Trojuholnik() {
        initUI();
    }

    private void initUI() {
        setSize(800, 600);
        setTitle("Sierpinski Triangle");
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        add(new Okno());
        setLocationRelativeTo(rootPane);
    }

    public static void main(String[] args) {
        SwingUtilities.invokeLater(new Runnable() {
            @Override
            public void run() {
                Trojuholnik trojuholnik = new Trojuholnik();
                trojuholnik.setVisible(true);
                Okno.inicializaciaTabulky();
                Okno.doplnenieTabulky();
                System.out.println(Arrays.deepToString(Okno.tabulka));
            }
        });
    }
}

The current result is as follows:

Answer 1

First of all, some basic suggestions:

, JFrame. , . , run() :

JFrame frame = new JFrame();
frame.setTitle("Sierpinski Triangle");
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.add( new Okno());
frame.pack();
frame.setLocationRelativeTo(null);
frame.setVisible(true);

Okno. .

getPreferredSize() , , . , , pack(). . , - Okno:

@Override
public Dimension getPreferredSize()
{
    return new Dimension(500, 500);
}

. , , , , 500 , Rectangle 50, 550x550, 500x500 .

, , (1, 1). , .