NullPointerException?

[Xillez]

Hello, I have no clue what to do with this error below.

Error:
Exception in thread "main" java.lang.NullPointerException
at main.Universe.<init>(Universe.java:31)
at main.Window.main(Window.java:91)

Window.java:

package main;
 
import java.awt.Color;
import java.awt.Graphics;
 
import javax.swing.JFrame;
import javax.swing.JMenuBar;
import javax.swing.JSlider;
import javax.swing.JToggleButton;
 
public class Window extends JFrame
{
    private static final long serialVersionUID = 1L;
 
    public static int wWidth = 1080;
    public static int wHeight = 720;
    public static boolean wVisible = false;
 
    //Input
    public static int nrMatter = 0;
    public static int nrAMatter = 0;
 
    public Window()
    {
        setTitle("Early universe sim");
        setSize(wWidth, wHeight);
        setVisible(true);
        setDefaultCloseOperation(EXIT_ON_CLOSE);
        getContentPane().setLayout(null);
 
        JMenuBar menuBar = new JMenuBar();
        setJMenuBar(menuBar);
 
        JToggleButton tglbtnRun = new JToggleButton("Run");
        menuBar.add(tglbtnRun);
 
        JSlider slider = new JSlider();
        slider.setMaximum(20);
        slider.setMinimum(10);
        slider.setExtent(1);
        menuBar.add(slider);
    }
 
    public void paint(Graphics g)
    {
        wVisible = true;
        while (wVisible)
        {
            //Check for wall collision before you move any objects
            for (int i = 0; i < Universe.id.length; i++)
            {
                if (Universe.xPos[i] == 0) { Universe.xSpeed[i] = Universe.xSpeed[i] * -1; }
                if (Universe.xPos[i] + Universe.cWidth == wWidth) { Universe.xSpeed[i] = Universe.xSpeed[i] * -1; }
                if (Universe.yPos[i] == 0) { Universe.ySpeed[i] = Universe.ySpeed[i] * -1; }
                if (Universe.yPos[i] + Universe.cHeight == wHeight) { Universe.ySpeed[i] = Universe.ySpeed[i] * -1; }
            }
 
            //Move
            for (int i = 0; i < Universe.id.length; i++)
            {
                Universe.xPos[i] = Universe.xPos[i] + Universe.xSpeed[i];
                Universe.yPos[i] = Universe.yPos[i] + Universe.ySpeed[i];
            }
 
            //Check for Collision here!
 
            //Painting the given amount of Matter at the current position
            for (int i = 0; i < Universe.nrMLoc; i++)
            {
                g.setColor(Color.RED);
                g.fillOval(Universe.xPos[i], Universe.yPos[i], Universe.cWidth, Universe.cHeight);
            }
 
            //Painting the given amount of Anti-Matter at the current position
            for(int i = Universe.nrAMLoc; i < Universe.nrAMLoc+Universe.nrMLoc; i++)
            {
                g.setColor(Color.BLUE);
                g.fillOval(Universe.xPos[i], Universe.yPos[i], Universe.cWidth, Universe.cHeight);
            }
 
            wVisible = false;
        }
    }
 
    public static void main(String[] args)
    {
        Universe.nrMLoc = 5;
        Universe.nrAMLoc = 5;
 
        //Initializing the early universes content
        Universe u = new Universe();
 
        //Construct window
        Window w = new Window();
    }
}
 

Universe.java:

package main;
 
import java.util.Random;
 
public class Universe 
{
    //Input
    public static int nrMLoc = 1;
    public static int nrAMLoc = 1;
    //Universe properties
    public static int cWidth = 25;
    public static int cHeight = 25;
    public static int maxX = Window.wWidth;
    public static int minX = 0;
    public static int maxY = Window.wHeight;
    public static int minY = 0;
    //Physics
    public static int gravity = 10;
    //Matter and Anti-Matter properties
    public static int[] id;
    public static int[] xPos;
    public static int[] yPos;
    public static int[] xSpeed;
    public static int[] ySpeed;
 
    public Universe()
    {
        Random r = new Random();
        for (int i = 0; i < nrMLoc+nrAMLoc; i++)
        {
            id[i] = i;
            xPos[i] = r.nextInt(Window.wWidth-2*cWidth)+cWidth;
            yPos[i] = r.nextInt(Window.wHeight-2*cHeight)+cHeight;
            xSpeed[i] = r.nextInt(30);
            ySpeed[i] = r.nextInt(30);
        }
    }
}
 

[Xillez]

Found it out. java is trying to convert from the data type null to int, and that is not possible. Just add integers in the array when you declare it.