I need to add collision to my tile map using MarteEngine. MarteEngine is built of of slick2D. Here's my tile generation code: Code:

public void render(GameContainer gc, StateBasedGame game, Graphics g) throws SlickException
   {
      for (int x = 0; x < 16; x++) {
         for (int y = 0; y < 16; y++) {
            map[x][y] = AIR;
               air.draw(x * GameWorld.tilesize, y * GameWorld.tilesize);
         }
      }

   for (int x = 0; x < 16; x++) {
         for (int y = 7; y < 8; y++) {
               map[x][y] = GRASS;
               grass.draw(x * tilesize, y * tilesize);
         }
      }

   for (int x = 0; x < 16; x++) {
         for (int y = 8; y < 10; y++) {
            map[x][y] = DIRT;
               dirt.draw(x * tilesize, y * tilesize);
         }
      }

   for (int x = 0; x < 16; x++) {
         for (int y = 10; y < 16; y++) {
            map[x][y] = STONE;
               stone.draw(x * tilesize, y * tilesize);   
         }
      }
      super.render(gc, game, g);


   }

And one of my tile classes (they're all the same, the image names are just different): Code:

package MarteEngine;

import org.newdawn.slick.Image;
import org.newdawn.slick.SlickException;

import it.randomtower.engine.entity.Entity;

public class Grass extends Entity {

   public static Image grass = null;

   public Grass(float x, float y) throws SlickException {
      super(x, y);

       grass = new Image("res/grass.png");

       setHitBox(0, 0, 50, 50);

       addType(SOLID);
   }

}

I tried to do it like this: Code:

for (int x = 0; x < 16; x++) {
         for (int y = 7; y < 8; y++) {
               map[x][y] = GRASS;
               Grass.grass.draw(x * tilesize, y * tilesize);
         }
      }

But it gave me a NullPointerException. No idea why, everything looks initialized right? I would be very grateful for some help!