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Quellcode

import java.awt.Color;
import java.awt.Graphics;
import java.util.Random;

import javax.swing.JPanel;


public class Pane extends JPanel {
    
    Cell[][] cells = new Cell[20][20];
    
    public Pane (){
        for (int x = 0;x <= cells.length; x++) {
            for (int y = 0;y <= cells[x].length; y++) {
                cells[x][y].isActive = false;
            }
        }
    }
    
    @Override
    public void paint(Graphics g) {
        super.paint(g);
        
        for (int x = 0;x <= cells.length; x++) {
            for (int y = 0;y <= cells[x].length; y++) {
                
                if (cells[x][y].isActive) {
                    g.setColor(Color.BLACK);
                } else {
                    g.setColor(Color.WHITE);
                }
                
                g.fillRect(x * Cell.CELL_SIZE, y * Cell.CELL_SIZE, Cell.CELL_SIZE, Cell.CELL_SIZE);
            }
        }
    }
}

class Cell {
    static int CELL_SIZE = 16;
    
    public boolean isActive = false;
    public boolean isNext = false;
    
    
}

Quellcode

Exception in thread "main" java.lang.NullPointerException
    at Pane.<init>(Pane.java:15)
    at Window.<init>(Window.java:13)
    at Window.main(Window.java:18)