使用JPanel时为Nullpointer

Question

for(creature a : horde.creatures){

总是抛出一个nullpointer，即使我之前定义过它：

private swarm horde;

这是群类：

public class swarm {
public creature creatures[] = new creature[100];
}

这只是一系列生物：

public class creature {
Random r = new Random();
public boolean hostile = r.nextBoolean();
public double[] velocity = new double[2];
public double[] position = new double[2];

整个窗口类：

package de.towdo.intellia;

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

final public class window {

JFrame frame;
DrawPanel drawPanel;

private swarm horde;

public static void main(String[] args){
    new window().go();
}

private void go(){
    frame = new JFrame("Window");
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

    drawPanel = new DrawPanel();

    frame.getContentPane().add(BorderLayout.CENTER, drawPanel);

    frame.setVisible(true);
    frame.setResizable(false);
    frame.setSize(500, 500);
    frame.setLocation(340, 55);
    moveIt();

}

class DrawPanel extends JPanel{

    public void paintComponent(Graphics g){
        for(creature a : horde.creatures){
            if(a.hostile){
                g.setColor(Color.RED);
            }
            else{
                g.setColor(Color.GREEN);
            }
            g.fillRect((int)a.position[0], (int)a.position[1], 1, 1);
        }
    }
}

private void moveIt(){
    while(true){
        try {
            Thread.sleep(10);
        }
        catch(Exception exc){
            exc.printStackTrace();
        }
        frame.repaint();
    }
}
}