import java.awt.Graphics;
import java.util.Date;
import java.applet.Applet;

public abstract class Sprite {

    // the screen position of the sprite
    protected double xpos = 0.0;
    protected double ypos = 0.0;

    // the velocity of the sprite
    protected int xvel = 0;
    protected int yvel = 0;

    // the dimentions of the sprite
    protected int xdim = 0;
    protected int ydim = 0;

    // the last time the position was calculated
    long lastupdate = 0;

    // class variables
    // the applet and its dimentions
    protected static Applet applet;
    protected static int appwidth  = 0;
    protected static int appheight = 0;

    // abstract methods: must be
    // implemented by sub-classes
    abstract void paint(Graphics g);

    // constructor
    public Sprite(Applet a) {
	appwidth  = a.getSize().width;
        appheight = a.getSize().height;
    }

    // calculate my new position
    public void update() {

        if (lastupdate == 0) { lastupdate = new Date().getTime(); }
        long thisupdate = new Date().getTime();
        long timediff = (thisupdate - lastupdate) / 10L;
        lastupdate = thisupdate;

        // boundaries for horizontal movement
        if (
            (xpos <= 0 && xvel < 0) ||
            ((xpos >= appwidth - xdim) && xvel > 0)
        ) {
            xvel = -xvel;
        }
        xpos += (double)(xvel*timediff);

        // boundaries for vertical movement
        if (
            (ypos <= 0 && yvel < 0) ||
            ((ypos >= appheight - ydim) && yvel > 0)
        ) {
            yvel = -yvel;
        }
        ypos += (double)(yvel*timediff);

    }

    // useful flags
    protected boolean is_visible = false;
    protected boolean is_active  = false;

    // accessor methods

}