# The Computer LanguageBenchmarks Game

## n-body Java #2 program

### source code

```/* The Computer Language Benchmarks Game
http://benchmarksgame.alioth.debian.org/

contributed by Mark C. Lewis
modified slightly by Chad Whipkey
*/

public final class nbody {
public static void main(String[] args) {
int n = Integer.parseInt(args[0]);

NBodySystem bodies = new NBodySystem();
System.out.printf("%.9f\n", bodies.energy());
for (int i=0; i<n; ++i)
System.out.printf("%.9f\n", bodies.energy());
}
}

final class NBodySystem {
private Body[] bodies;

public NBodySystem(){
bodies = new Body[]{
Body.sun(),
Body.jupiter(),
Body.saturn(),
Body.uranus(),
Body.neptune()
};

double px = 0.0;
double py = 0.0;
double pz = 0.0;
for(int i=0; i < bodies.length; ++i) {
px += bodies[i].vx * bodies[i].mass;
py += bodies[i].vy * bodies[i].mass;
pz += bodies[i].vz * bodies[i].mass;
}
bodies[0].offsetMomentum(px,py,pz);
}

public void advance(double dt) {
for(int i=0; i < bodies.length; ++i) {
Body iBody = bodies[i];
for(int j=i+1; j < bodies.length; ++j) {
double dx = iBody.x - bodies[j].x;
double dy = iBody.y - bodies[j].y;
double dz = iBody.z - bodies[j].z;

double dSquared = dx * dx + dy * dy + dz * dz;
double distance = Math.sqrt(dSquared);
double mag = dt / (dSquared * distance);

iBody.vx -= dx * bodies[j].mass * mag;
iBody.vy -= dy * bodies[j].mass * mag;
iBody.vz -= dz * bodies[j].mass * mag;

bodies[j].vx += dx * iBody.mass * mag;
bodies[j].vy += dy * iBody.mass * mag;
bodies[j].vz += dz * iBody.mass * mag;
}
}

for ( Body body : bodies) {
body.x += dt * body.vx;
body.y += dt * body.vy;
body.z += dt * body.vz;
}
}

public double energy(){
double dx, dy, dz, distance;
double e = 0.0;

for (int i=0; i < bodies.length; ++i) {
Body iBody = bodies[i];
e += 0.5 * iBody.mass *
( iBody.vx * iBody.vx
+ iBody.vy * iBody.vy
+ iBody.vz * iBody.vz );

for (int j=i+1; j < bodies.length; ++j) {
Body jBody = bodies[j];
dx = iBody.x - jBody.x;
dy = iBody.y - jBody.y;
dz = iBody.z - jBody.z;

distance = Math.sqrt(dx*dx + dy*dy + dz*dz);
e -= (iBody.mass * jBody.mass) / distance;
}
}
return e;
}
}

final class Body {
static final double PI = 3.141592653589793;
static final double SOLAR_MASS = 4 * PI * PI;
static final double DAYS_PER_YEAR = 365.24;

public double x, y, z, vx, vy, vz, mass;

public Body(){}

static Body jupiter(){
Body p = new Body();
p.x = 4.84143144246472090e+00;
p.y = -1.16032004402742839e+00;
p.z = -1.03622044471123109e-01;
p.vx = 1.66007664274403694e-03 * DAYS_PER_YEAR;
p.vy = 7.69901118419740425e-03 * DAYS_PER_YEAR;
p.vz = -6.90460016972063023e-05 * DAYS_PER_YEAR;
p.mass = 9.54791938424326609e-04 * SOLAR_MASS;
return p;
}

static Body saturn(){
Body p = new Body();
p.x = 8.34336671824457987e+00;
p.y = 4.12479856412430479e+00;
p.z = -4.03523417114321381e-01;
p.vx = -2.76742510726862411e-03 * DAYS_PER_YEAR;
p.vy = 4.99852801234917238e-03 * DAYS_PER_YEAR;
p.vz = 2.30417297573763929e-05 * DAYS_PER_YEAR;
p.mass = 2.85885980666130812e-04 * SOLAR_MASS;
return p;
}

static Body uranus(){
Body p = new Body();
p.x = 1.28943695621391310e+01;
p.y = -1.51111514016986312e+01;
p.z = -2.23307578892655734e-01;
p.vx = 2.96460137564761618e-03 * DAYS_PER_YEAR;
p.vy = 2.37847173959480950e-03 * DAYS_PER_YEAR;
p.vz = -2.96589568540237556e-05 * DAYS_PER_YEAR;
p.mass = 4.36624404335156298e-05 * SOLAR_MASS;
return p;
}

static Body neptune(){
Body p = new Body();
p.x = 1.53796971148509165e+01;
p.y = -2.59193146099879641e+01;
p.z = 1.79258772950371181e-01;
p.vx = 2.68067772490389322e-03 * DAYS_PER_YEAR;
p.vy = 1.62824170038242295e-03 * DAYS_PER_YEAR;
p.vz = -9.51592254519715870e-05 * DAYS_PER_YEAR;
p.mass = 5.15138902046611451e-05 * SOLAR_MASS;
return p;
}

static Body sun(){
Body p = new Body();
p.mass = SOLAR_MASS;
return p;
}

Body offsetMomentum(double px, double py, double pz){
vx = -px / SOLAR_MASS;
vy = -py / SOLAR_MASS;
vz = -pz / SOLAR_MASS;
return this;
}
}

```

### notes, command-line, and program output

```NOTES:
64-bit Ubuntu quad core
java 10 2018-03-20
Java(TM) SE Runtime Environment 18.3 (build 10+46)
Java HotSpot(TM) 64-Bit Server VM 18.3 (build 10+46, mixed mode)

Wed, 21 Mar 2018 18:18:34 GMT

MAKE:
mv nbody.java-2.java nbody.java
/opt/src/jdk-10/bin/javac -d .  nbody.java

1.06s to complete and log all make actions

COMMAND LINE:
/opt/src/jdk-10/bin/java   nbody 50000000

PROGRAM OUTPUT:
-0.169075164
-0.169059907
```