Killer Game Programming in Java

The Player Sprite

PlayerSprite represents the player and is a subclass of TiledSprite. The statechart for PlayerSprite in Figure 13-14 shows that the sprite performs three concurrent activities.

The move() and tryPickup() transitions are triggered by the user from the keyboard. The hitByAlien() transition is initiated by the WorldDisplay object when an alien tells it that it has hit the player.

Tip

The transitions in Figure 13-14 are labeled with method names; this is a practice that I'll use when there's a direct mapping from a transition to a method call. This makes it easier to see the mapping from the statechart to the corresponding code.

Figure 13-14. PlayerSprite statechart

Moving (and Standing Still)

A PlayerSprite tries to move when the user presses one of the quadrant keys (9, 3, 1, or 7):

    public void move(int quad)
    {
      Point newPt = tryMove(quad);
      if (newPt == null) {   // move not possible
        clipsLoader.play("slap", false);
        standStill();
      }
      else {    // move is possible
        setTileLoc(newPt);    // update the sprite's tile location
        if (quad == NE)
          setImage("ne");
        else if (quad == SE)
          setImage("se");
        else if (quad == SW)

          setImage("sw");
        else // quad == NW
          setImage("nw");
        world.playerHasMoved(newPt, quad);
      }
    }  // end of move()

The attempt is handled by TiledSprite's inherited tryMove() method, and the sprite's tile location is updated if it's successful. The move is dressed up with an image change ...