Killer Game Programming in Java

A Sprite on a Tile

A TiledSprite represents a sprite's position using tile coordinates (xTile, yTile); its most important method allows a sprite to move from its current tile to an adjacent one using a compass direction (quadrant): NE, SE, SW, NW. One assumption of TiledSprite is that a sprite cannot move around inside a tile—the sprite can only step from one tile to another.

The constructor initializes a sprite's tile position after checking its validity with WorldDisplay:

    protected int xTile, yTile;    // tile coordinate for the sprite
    protected WorldDisplay world;


    public TiledSprite(int x, int y, int w, int h,
                             ImagesLoader imsLd, String name,
                             WorldDisplay wd)
    { super(0, 0, w, h, imsLd, name);

      setStep(0, 0);      // no movement
      world = wd;

      if (!world.validTileLoc(x, y)) {  // is tile (x,y) valid
        System.out.println("Alien tile location (" + x + "," + y +
                                          ") not valid; using (0,0)");
        x = 0; y = 0;
      }
      xTile = x; yTile = y;
    }  // end of TiledSprite()

Moving to Another Tile

AlienTiles' staggered tile layout means that the coordinates of the four tiles adjacent to the current one are obtained in different ways, depending on if the current tile is on an even or odd row. Figures 13-12 and 13-13 show examples of the two possibilities.

The highlighted tile in Figure 13-12 is in row 3 (odd), and the one in Figure 13-13 is in row 2 (even). The coordinates of the adjacent tiles are calculated differently in these two cases.

Figure 13-12. Moving from tile (1,3)

Figure 13-13. Moving from tile (1,2)

tryMove() ...