doc/next/examples/spacewar/SpaceObject.java

/*

Copyright (c) Xerox Corporation 1998-2002.  All rights reserved.

Use and copying of this software and preparation of derivative works based
upon this software are permitted.  Any distribution of this software or
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This software is made available AS IS, and Xerox Corporation makes no warranty
about the software, its performance or its conformity to any specification.

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SpaceObject.java
Part of the Spacewar system.

*/

package spacewar;


/**
 * SpaceObjects are objects that float around in space.  They support the
 * minimal SpaceObject protocol, having to do with position, velocity,
 * size and liveness.  They are constructed with game, position, velocity
 * and size.  When constructed, a spaceobject adds itself to the registry.
 *
 * When it dies, a spaceobject removes itself from the registry.  But note
 * that it doesn't decide when to die, subclasses do that.
 *
 * The display aspects actually draw the space object on the screen and say
 * how much space it takes up there.
 */
abstract class SpaceObject {

    private Game    game;
    private double  xPos, yPos, oldXPos, oldYPos, xVel, yVel;
    private boolean alive;

    SpaceObject (Game theGame, double xP, double yP, double xV, double yV) {
        game = theGame;
        xPos = xP;
        yPos = yP;
        oldXPos = xP;
        oldYPos = yP;
        xVel = xV;
        yVel = yV;

        alive = true;
        getGame().getRegistry().register(this);
    }

    Game getGame()      { return game; }

    double getXPos()    { return xPos; }
    double getYPos()    { return yPos; }

    double getOldXPos() { return oldXPos; }
    double getOldYPos() { return oldYPos; }

    double getXVel()    { return xVel; }
    double getYVel()    { return yVel; }

    void setXVel (double n) { xVel = n; }
    void setYVel (double n) { yVel = n; }

    boolean isAlive()          { return alive; }
    void setIsAlive(boolean n) { alive = n; }


    /**
     * Move 1 unit of time's worth of distance.  I.e. increment xPos by xVel
     * and yPos by yVel.  If we move off an edge of the screen move us back
     * in the opposite edge.
     */
    void clockTick() {
        oldXPos = xPos;
        oldYPos = yPos;
        xPos = (xPos + xVel) % getGame().getWidth();
        if(xPos < 0)
            xPos += getGame().getWidth();
        yPos = (yPos + yVel) % getGame().getHeight();
        if(yPos < 0)
            yPos += getGame().getHeight();
    }

    void accelerate(double dXVel, double dYVel) {
        xVel += dXVel;
        yVel += dYVel;
    }

    void die() {
        getGame().getRegistry().unregister(this);
    }

    abstract int getSize();

    /** resolve the effects of colliding with a space object.
     *  @param obj the space object that this object is colliding with.
     */
    abstract void handleCollision(SpaceObject obj);
}