/**

* A polygon that moves around the screen.

*/

public class MovingPolygon extends RegularPolygon {

private int dx;

private int dy;

/**

* Constructs a moving polygon.

*

* @param nsides the number of sides

* @param length length of each side

*/

public MovingPolygon(int nsides, int length) {

super(nsides, length);

this.dx = 10;

this.dy = 5;

}

/**

* @param dx how many pixels to move left/right

*/

public void setDx(int dx) {

this.dx = dx;

}

/**

* @param dy how many pixels to move up/down

*/

public void setDy(int dy) {

this.dy = dy;

}

@Override

public void act() {

// edge detection

for (int i = 0; i < npoints; i++) {

if (xpoints[i] < 0 || xpoints[i] > 800) {

dx *= -1;

break;

}

}

for (int i = 0; i < npoints; i++) {

if (ypoints[i] < 0 || ypoints[i] > 600) {

dy *= -1;

break;

}

}

// move one step

translate(dx, dy);

}

}