dot random movement in java

Image img = ImageViewer.getImage();
    int width=img.getWidth();
    int height=img.getHeight();
    int p=width*1/2;
    int q=height*1/2;
int []a= {1,-1,-1,-1,1,-1,-1,1,-1,1,1,1,1,1,-1,-1,1,-1};//A random array with 1 or -1


    for(int i:a) {
        for(int x=p;x<width&&x>0;x+=i) {
            for(int y=q;y<height&&y>0;y+=i) {
                img.setPixelColor(x, y, Color.BLACK);
            }
        }
    }

I want to draw random dots starting from the center of a blank image. The dots should travel randomly up, down, left or right until one leaves the border of the image. Here's what I have done so far, which doesn't work at all.