Killer Game Programming in Java

The Alien Sprite

AlienSprite implements the basic behavior of an alien sprite and is subclassed to create the AlienAStarSprite and AlienQuadSprite classes. AlienSprite is a subclass of TiledSprite.

Alien behavior can best be understood by considering the statechart in Figure 13-15.

The plan move state is entered by the WorldDisplay object, notifying the alien that the player has moved. This gives it the opportunity to recalculate its current direction or destination, but the precise algorithm will vary from one AlienSprite subclass to another.

The other activity is the usual update/draw cycle driven by the animation loop in AlienTilesPanel. The alien tries to hit the player while in the attack state. A successful hit is reported to the WorldDisplay object, and the alien stays where it is. Otherwise, the alien updates its position, in the hope of getting closer to the player. In the draw state, the sprite's tile coordinates are mapped to a pixel location and the sprite's image is rendered.

Figure 13-15. Alien statechart

Responding to a player's movement is sprite-specific, so playerHasMoved() is empty in AlienSprite:

    public void playerHasMoved(Point playerLoc)
    { }

PlayerLoc contains the current tile coordinates for the PlayerSprite object.

Updating the AlienSprite

The attack, stationary, and move states are encapsulated in update():

 // globals private final static int UPDATE_FREQ = 30; ...