code: private static Double getCollisionX(CollisionCategory category, TileCollision tileCollision, double x, double y)
      {
          if (Axis.X == category.getAxis())
          {
              return tileCollision.getCollisionX(category, x, y);
          }
          return null;
      }
docs: Get the horizontal collision from current location.
  
  @param category The collision category.
  @param tileCollision The current tile collision.
  @param x The current horizontal location.
  @param y The current vertical location.
  @return The computed horizontal collision.