Universe.java

Below is the syntax highlighted version of Universe.java from §3.4 Case Study: N-Body Simulation.

/******************************************************************************
 *  Compilation:  javac Universe.java
 *  Execution:    java Universe dt < input.txt
 *  Dependencies: Body.java Vector.java StdIn.java StdDraw.java
 *  Datafiles:    https://introcs.cs.princeton.edu/java/34nbody/2body.txt
 *                https://introcs.cs.princeton.edu/java/34nbody/3body.txt
 *                https://introcs.cs.princeton.edu/java/34nbody/4body.txt
 *                https://introcs.cs.princeton.edu/java/34nbody/2bodyTiny.txt
 *
 *  This data-driven program simulates motion in the universe defined
 *  by the standard input stream, increasing time at the rate on the
 *  command line.
 *
 *  %  java Universe 25000 < 4body.txt
 *
 *
 ******************************************************************************/

public class Universe {
    private final int n;             // number of bodies
    private final Body[] bodies;     // array of n bodies

    // read universe from standard input
    public Universe() {

        // number of bodies
        n = StdIn.readInt();

        // the set scale for drawing on screen
        double radius = StdIn.readDouble();
        StdDraw.setXscale(-radius, +radius);
        StdDraw.setYscale(-radius, +radius);

        // read in the n bodies
        bodies = new Body[n];
        for (int i = 0; i < n; i++) {
            double rx   = StdIn.readDouble();
            double ry   = StdIn.readDouble();
            double vx   = StdIn.readDouble();
            double vy   = StdIn.readDouble();
            double mass = StdIn.readDouble();
            double[] position = { rx, ry };
            double[] velocity = { vx, vy };
            Vector r = new Vector(position);
            Vector v = new Vector(velocity);
            bodies[i] = new Body(r, v, mass);
        }
    }

    // increment time by dt units, assume forces are constant in given interval
    public void increaseTime(double dt) {

        // initialize the forces to zero
        Vector[] f = new Vector[n];
        for (int i = 0; i < n; i++) {
            f[i] = new Vector(new double[2]);
        }

        // compute the forces
        for (int i = 0; i < n; i++) {
            for (int j = 0; j < n; j++) {
                if (i != j) {
                    f[i] = f[i].plus(bodies[i].forceFrom(bodies[j]));
                }
            }
        }

        // move the bodies
        for (int i = 0; i < n; i++) {
            bodies[i].move(f[i], dt);
        }
    }

    // draw the n bodies
    public void draw() {
        for (int i = 0; i < n; i++) {
            bodies[i].draw();
        }
    }


    // client to simulate a universe
    public static void main(String[] args) {
        Universe newton = new Universe();
        double dt = Double.parseDouble(args[0]);
        StdDraw.enableDoubleBuffering();
        while (true) {
            StdDraw.clear();
            newton.increaseTime(dt);
            newton.draw();
            StdDraw.show();
            StdDraw.pause(10);
        }
    }
}