NBody.java

public class NBody {
    public static double readRadius(String file)
    {
        In in = new In(file);
        in.readDouble();
        return in.readDouble();
    }

    public static Planet [] readPlanets(String file)
    {
        In in = new In(file);
        Planet [] ps = new Planet[in.readInt()];
        in.readDouble();
        for(int i=0; i<ps.length; ++i)
        {
            ps[i] = new Planet(in.readDouble(),in.readDouble(), in.readDouble(),
                        in.readDouble(), in.readDouble(), in.readString());
        }
        return ps;
    }

    public static void main (String [] args)
    {
        Double T = Double.parseDouble(args[0]), 
               dt = Double.parseDouble(args[1]);
        String filename = args[2];
        double radius = readRadius(filename);

        StdDraw.enableDoubleBuffering();
        StdDraw.setScale(0-radius,radius);
        StdDraw.clear();
        StdDraw.picture(0,0, "images/starfield.jpg");

        Planet [] ps = readPlanets(filename);
        for(Planet p : ps)
        {
            p.draw();
        }

        double t = 0;
        while(t < T)
        {
            double [] xForces = new double [ps.length];
            double [] yForces = new double [ps.length];

            for(int i=0; i < ps.length; ++i)
            {
                xForces[i] = ps[i].calcNetForceExertedByX(ps);
                yForces[i] = ps[i].calcNetForceExertedByY(ps);
            }    

            StdDraw.picture(0,0, "images/starfield.jpg");
            for(int i=0; i<ps.length; ++i)
            {
                ps[i].update(dt, xForces[i], yForces[i]);   
                ps[i].draw();
            }
            StdDraw.show(); 
            StdDraw.pause(20);   
            t += dt;
        }
        StdOut.printf("%d\n", ps.length);
        StdOut.printf("%.2e\n", radius);
        for (int i = 0; i < ps.length; i++) {
            StdOut.printf("%11.4e %11.4e %11.4e %11.4e %11.4e %12s\n",
                  ps[i].xxPos, ps[i].yyPos, ps[i].xxVel,
                  ps[i].yyVel, ps[i].mass, ps[i].imgFileName);   
        }
    } 
};