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.getMass()
* (iBody.getVX() * iBody.getVX()
+ iBody.getVY() * iBody.getVY()
+ iBody.getVZ() * iBody.getVZ());
for (int j = i + 1; j < bodies.length; ++j) {
Body jBody = bodies[j];
dx = iBody.getX() - jBody.getX();
dy = iBody.getY() - jBody.getY();
dz = iBody.getZ() - jBody.getZ();
distance = Math.sqrt(dx * dx + dy * dy + dz * dz);
e -= (iBody.getMass() * jBody.getMass()) / distance;
}
}
return e;
}
public void advance(final double dt) {
for (int i = 0; i < bodies.length; ++i) {
Body iBody = bodies[i];
for (int j = i + 1; j < bodies.length; ++j) {
Body jBody = bodies[j];
double dx = iBody.getX() - jBody.getX();
double dy = iBody.getY() - jBody.getY();
double dz = iBody.getZ() - jBody.getZ();
double dSquared = dx * dx + dy * dy + dz * dz;
double distance = Math.sqrt(dSquared);
double mag = dt / (dSquared * distance);
iBody.setVX(iBody.getVX() - (dx * jBody.getMass() * mag));
iBody.setVY(iBody.getVY() - (dy * jBody.getMass() * mag));
iBody.setVZ(iBody.getVZ() - (dz * jBody.getMass() * mag));
jBody.setVX(jBody.getVX() + (dx * iBody.getMass() * mag));
jBody.setVY(jBody.getVY() + (dy * iBody.getMass() * mag));
jBody.setVZ(jBody.getVZ() + (dz * iBody.getMass() * mag));
}
}
for (Body body : bodies) {
body.setX(body.getX() + dt * body.getVX());
body.setY(body.getY() + dt * body.getVY());
body.setZ(body.getZ() + dt * body.getVZ());
}
}
public Body[] createBodies() {
Body[] bodies = new Body[5];
bodies[0] = Body.sun();
bodies[1] = Body.jupiter();
bodies[2] = Body.saturn();
bodies[3] = Body.uranus();
bodies[4] = Body.neptune();
double px = 0.0;
double py = 0.0;
double pz = 0.0;
for (Body b : bodies) {
px += b.getVX() * b.getMass();
py += b.getVY() * b.getMass();
pz += b.getVZ() * b.getMass();
}
bodies[0].offsetMomentum(px, py, pz);
return bodies;
}