Examples:SpaceInvaders AlienEntity

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package org.lwjgl.examples.spaceinvaders;

/**
 * An entity which represents one of our space invader aliens.
 * 
 * @author Kevin Glass
 * @author Brian Matzon
 */
public class AlienEntity extends Entity {

	/** Movement made downwards when a border is hit */
	private static final int	DOWNWARD_MOVEMENT	= 10;

	/** Border at which player dies */
	private static final int	BOTTOM_BORDER			= 570;

	/** Right border at which to shift direction */
	private static final int	RIGHT_BORDER			= 750;

	/** Left border at which to shift direction */
	private static final int	LEFT_BORDER				= 10;

	/** The speed at which the alient moves horizontally */
	private float							moveSpeed					= 75;

	/** The game in which the entity exists */
	private Game							game;

	/** The animation frames */
	private Sprite[]					frames						= new Sprite[4];

	/** The time since the last frame change took place */
	private long							lastFrameChange;

	/** The frame duration in milliseconds, i.e. how long any given frame of animation lasts */
	private long							frameDuration			= 250;

	/** The current frame of animation being displayed */
	private int								frameNumber;

	/**
	 * Create a new alien entity
	 * 
	 * @param game The game in which this entity is being created
	 * @param x The intial x location of this alien
	 * @param y The intial y location of this alient
	 */
	public AlienEntity(Game game, int x, int y) {
		super(game.getSprite("alien.gif"), x, y);

		// setup the animatin frames
		frames[0] = sprite;
		frames[1] = game.getSprite("alien2.gif");
		frames[2] = sprite;
		frames[3] = game.getSprite("alien3.gif");

		this.game = game;
		dx = -moveSpeed;
	}

	/**
	 * Request that this alien moved based on time elapsed
	 * 
	 * @param delta The time that has elapsed since last move
	 */
	public void move(long delta) {
		// since the move tells us how much time has passed
		// by we can use it to drive the animation, however
		// its the not the prettiest solution
		lastFrameChange += delta;

		// if we need to change the frame, update the frame number
		// and flip over the sprite in use
		if (lastFrameChange > frameDuration) {
			// reset our frame change time counter
			lastFrameChange = 0;

			// update the frame
			frameNumber++;
			if (frameNumber >= frames.length) {
				frameNumber = 0;
			}

			sprite = frames[frameNumber];
		}

		// if we have reached the left hand side of the screen and
		// are moving left then request a logic update 
		if ((dx < 0) && (x < LEFT_BORDER)) {
			game.updateLogic();
		}
		// and vice vesa, if we have reached the right hand side of 
		// the screen and are moving right, request a logic update
		if ((dx > 0) && (x > RIGHT_BORDER)) {
			game.updateLogic();
		}

		// proceed with normal move
		super.move(delta);
	}

	/**
	 * Update the game logic related to aliens
	 */
	public void doLogic() {
		// swap over horizontal movement and move down the
		// screen a bit
		dx = -dx;
		y += DOWNWARD_MOVEMENT;

		// if we've reached the bottom of the screen then the player
		// dies
		if (y > BOTTOM_BORDER) {
			game.notifyDeath();
		}
	}

	/**
	 * Notification that this alien has collided with another entity
	 * 
	 * @param other The other entity
	 */
	public void collidedWith(Entity other) {
		// collisions with aliens are handled elsewhere
	}
}