Moving problem (Side-scroller)

Hello !

I currently have a problem with my Side-scroller. (I'm using SLICK2D)
My player moves correctly(the background in fact), and the collisions are working correctly too.
But I don't really know how to move correctly my monster entity, because they don't move with the background...

Here are my codes:

Player.java :

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package com.poulpicious.gtw.entities;

import org.newdawn.slick.Color;
import org.newdawn.slick.GameContainer;
import org.newdawn.slick.Graphics;
import org.newdawn.slick.Input;
import org.newdawn.slick.geom.Rectangle;
import org.newdawn.slick.state.StateBasedGame;

public class Player extends LivingEntity {

   public Player(float x, float y, float width, float height) {
      super(x, y, width, height);
      screenXPos = 400;
   }

   @Override
   public void render(GameContainer gc, StateBasedGame sbg, Graphics g) {
      g.setColor(Color.white);
      g.draw(new Rectangle(screenXPos - width / 2 + 1, yPos, width, height));
   }

   @Override
   public void update(GameContainer gc, StateBasedGame sbg, int delta, Input input) {
      if (input.isKeyDown(Input.KEY_Q)) {
         xDir = -2;
      }
      if (input.isKeyDown(Input.KEY_D)) {
         xDir = 2;
      }
      if (input.isKeyDown(Input.KEY_LSHIFT)) {
         xDir *= 1.4;
      }
      if (input.isKeyDown(Input.KEY_SPACE) && hasJump) {
         yDir = (float) -(4);
         hasJump = false;
         startJumpY = yPos;
      }

      startX = xPos;
      startY = yPos;
      startXDir = xDir;
      startYDir = yDir;

      collidingTop = false;

      moveBounds();
      gravity();
      move();
      handleCollisions();
      xDir = 0;
   }

   public void moveBounds() {
      bounds.setLocation(xPos + xDir, yPos + yDir);
   }

   public void move() {
      xPos += xDir;
      yPos += yDir;
   }

   public void gravity() {
      yDir += 0.06;
   }

   private void handleCollisions() {
      for (Rectangle rec : blocks) {

         Rectangle rect = new Rectangle(rec.getX() - screenXPos + width / 2 - 1, rec.getY(), rec.getWidth(), rec.getHeight());

         float[] points = bounds.getPoints();

         if (rect.contains(points[4] - 2, points[5] + 2) || rect.contains(points[6] + 2, points[7] + 2)) {
            yPos = startY;
            yDir = 0;
            hasJump = true;
            collidingTop = true;
         }

         if (rect.intersects(getBounds()) && collidingTop == false) {
            xPos = startX;
            xDir = startXDir;
         }
      }
   }

   public float getRealXPos() {
      return screenXPos + width / 2 - 1;
   }
}

MobEntity.java :

package com.poulpicious.gtw.entities;

import org.newdawn.slick.GameContainer;
import org.newdawn.slick.Graphics;
import org.newdawn.slick.Input;
import org.newdawn.slick.geom.Rectangle;
import org.newdawn.slick.state.StateBasedGame;

public class MobEntity extends LivingEntity {

   private Player thePlayer;

   public MobEntity(Player player, float x, float y, float width, float height) {
      super(x, y, width, height);
      thePlayer = player;
      isAlive = true;
      screenXPos = x;
   }

   @Override
   public void render(GameContainer gc, StateBasedGame sbg, Graphics g) {
      g.draw(new Rectangle(xPos - screenXPos, yPos, width, height));
   }

   @Override
   public void update(GameContainer gc, StateBasedGame sbg, int delta, Input input) {
      if (thePlayer.getRealXPos() < xPos - screenXPos) {
         xDir = -2;
      }
      if (thePlayer.getRealXPos() > xPos - screenXPos) {
         xDir = 2;
      }
      startX = xPos;
      startY = yPos;
      startXDir = xDir;
      startYDir = yDir;

      collidingTop = false;

      moveBounds();
      gravity();
      move();
      handleCollisions();
      xDir = 0;
   }

   public void moveBounds() {
      bounds.setLocation(xPos + xDir, yPos + yDir);
      System.out.println(bounds.getX());
   }

   public void move() {
      xPos += xDir;
      yPos += yDir;
   }

   public void gravity() {
      yDir += 0.06;
   }

   private void handleCollisions() {
      for (Rectangle rec : blocks) {

         Rectangle rect = new Rectangle(rec.getX() - screenXPos, rec.getY(), rec.getWidth(), rec.getHeight());
         float[] points = bounds.getPoints();

         if (rect.contains(points[4] - 2, points[5] + 2) || rect.contains(points[6] + 2, points[7] + 2)) {
            yPos = startY;
            yDir = 0;
            hasJump = true;
            collidingTop = true;
         }

         if (rect.intersects(getBounds()) && collidingTop == false) {
            xPos = startX;
            xDir = startXDir;
         }
      }
   }

   public boolean isAlive() {
      return isAlive;
   }

}