Task: N-body problem
C: #include<stdlib.h>
#include<stdio.h>
#include<math.h>

typedef struct{
	double x,y,z;
}vector;

int bodies,timeSteps;
double *masses,GravConstant;
vector *positions,*velocities,*accelerations;

vector addVectors(vector a,vector b){
	vector c = {a.x+b.x,a.y+b.y,a.z+b.z};
	
	return c;
}

vector scaleVector(double b,vector a){
	vector c = {b*a.x,b*a.y,b*a.z};
	
	return c;
}

vector subtractVectors(vector a,vector b){
	vector c = {a.x-b.x,a.y-b.y,a.z-b.z};
	
	return c;
}

double mod(vector a){
	return sqrt(a.x*a.x + a.y*a.y + a.z*a.z);
}

void initiateSystem(char* fileName){
	int i;
	FILE* fp = fopen(fileName,"r");
	
	fscanf(fp,"%lf%d%d",&GravConstant,&bodies,&timeSteps);

	masses = (double*)malloc(bodies*sizeof(double));
	positions = (vector*)malloc(bodies*sizeof(vector));
	velocities = (vector*)malloc(bodies*sizeof(vector));
	accelerations = (vector*)malloc(bodies*sizeof(vector));
	
	for(i=0;i<bodies;i++){
		fscanf(fp,"%lf",&masses[i]);
		fscanf(fp,"%lf%lf%lf",&positions[i].x,&positions[i].y,&positions[i].z);
		fscanf(fp,"%lf%lf%lf",&velocities[i].x,&velocities[i].y,&velocities[i].z);
	}
	
	fclose(fp);
}

void resolveCollisions(){
	int i,j;
	
	for(i=0;i<bodies-1;i++)
		for(j=i+1;j<bodies;j++){
			if(positions[i].x==positions[j].x && positions[i].y==positions[j].y && positions[i].z==positions[j].z){
				vector temp = velocities[i];
				velocities[i] = velocities[j];
				velocities[j] = temp;
			}
		}
}

void computeAccelerations(){
	int i,j;
	
	for(i=0;i<bodies;i++){
		accelerations[i].x = 0;
		accelerations[i].y = 0;
		accelerations[i].z = 0;
		for(j=0;j<bodies;j++){
			if(i!=j){
				accelerations[i] = addVectors(accelerations[i],scaleVector(GravConstant*masses[j]/pow(mod(subtractVectors(positions[i],positions[j])),3),subtractVectors(positions[j],positions[i])));
			}
		}
	}
}

void computeVelocities(){
	int i;
	
	for(i=0;i<bodies;i++)
		velocities[i] = addVectors(velocities[i],accelerations[i]);
}

void computePositions(){
	int i;
	
	for(i=0;i<bodies;i++)
		positions[i] = addVectors(positions[i],addVectors(velocities[i],scaleVector(0.5,accelerations[i])));
}

void simulate(){
	computeAccelerations();
	computePositions();
	computeVelocities();
	resolveCollisions();
}

int main(int argC,char* argV[])
{
	int i,j;
	
	if(argC!=2)
		printf("Usage : %s <file name containing system configuration data>",argV[0]);
	else{
		initiateSystem(argV[1]);
		printf("Body   :     x              y               z           |           vx              vy              vz   ");
		for(i=0;i<timeSteps;i++){
			printf("\nCycle %d\n",i+1);
			simulate();
			for(j=0;j<bodies;j++)
				printf("Body %d : %lf\t%f\t%lf\t|\t%lf\t%lf\t%lf\n",j+1,positions[j].x,positions[j].y,positions[j].z,velocities[j].x,velocities[j].y,velocities[j].z);
		}
	}
	return 0;
}

Java: import java.io.IOException;
import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.Paths;
import java.util.Arrays;
import java.util.List;

public class NBodySim {
    private static class Vector3D {
        double x, y, z;

        public Vector3D(double x, double y, double z) {
            this.x = x;
            this.y = y;
            this.z = z;
        }

        public Vector3D plus(Vector3D rhs) {
            return new Vector3D(x + rhs.x, y + rhs.y, z + rhs.z);
        }

        public Vector3D minus(Vector3D rhs) {
            return new Vector3D(x - rhs.x, y - rhs.y, z - rhs.z);
        }

        public Vector3D times(double s) {
            return new Vector3D(s * x, s * y, s * z);
        }

        public double mod() {
            return Math.sqrt(x * x + y * y + z * z);
        }
    }

    private static final Vector3D origin = new Vector3D(0, 0, 0);

    private static class NBody {
        private double gc;
        private int bodies;
        public final int timeSteps;
        private double[] masses;
        private Vector3D[] positions;
        private Vector3D[] velocities;
        private Vector3D[] accelerations;

        public NBody(String fileName) throws IOException {
            Path path = Paths.get(fileName);
            List<String> lines = Files.readAllLines(path);

            String[] gbt = lines.get(0).split(" ");
            gc = Double.parseDouble(gbt[0]);
            bodies = Integer.parseInt(gbt[1]);
            timeSteps = Integer.parseInt(gbt[2]);
            masses = new double[bodies];
            positions = new Vector3D[bodies];
            Arrays.fill(positions, origin);
            velocities = new Vector3D[bodies];
            Arrays.fill(velocities, origin);
            accelerations = new Vector3D[bodies];
            Arrays.fill(accelerations, origin);
            for (int i = 0; i < bodies; ++i) {
                masses[i] = Double.parseDouble(lines.get(i * 3 + 1));
                positions[i] = decompose(lines.get(i * 3 + 2));
                velocities[i] = decompose(lines.get(i * 3 + 3));
            }
            System.out.printf("Contents of %s\n", fileName);
            for (String line : lines) {
                System.out.println(line);
            }
            System.out.println();
            System.out.print("Body   :      x          y          z    |");
            System.out.println("     vx         vy         vz");
        }

        private Vector3D decompose(String line) {
            String[] xyz = line.split(" ");
            double x = Double.parseDouble(xyz[0]);
            double y = Double.parseDouble(xyz[1]);
            double z = Double.parseDouble(xyz[2]);
            return new Vector3D(x, y, z);
        }

        private void resolveCollisions() {
            for (int i = 0; i < bodies; ++i) {
                for (int j = i + 1; j < bodies; ++j) {
                    if (positions[i].x == positions[j].x
                        && positions[i].y == positions[j].y
                        && positions[i].z == positions[j].z) {
                        Vector3D temp = velocities[i];
                        velocities[i] = velocities[j];
                        velocities[j] = temp;
                    }
                }
            }
        }

        private void computeAccelerations() {
            for (int i = 0; i < bodies; ++i) {
                accelerations[i] = origin;
                for (int j = 0; j < bodies; ++j) {
                    if (i != j) {
                        double temp = gc * masses[j] / Math.pow((positions[i].minus(positions[j])).mod(), 3);
                        accelerations[i] = accelerations[i].plus(positions[j].minus(positions[i]).times(temp));
                    }
                }
            }
        }

        private void computeVelocities() {
            for (int i = 0; i < bodies; ++i) {
                velocities[i] = velocities[i].plus(accelerations[i]);
            }
        }

        private void computePositions() {
            for (int i = 0; i < bodies; ++i) {
                positions[i] = positions[i].plus(velocities[i]).plus(accelerations[i].times(0.5));
            }
        }

        public void simulate() {
            computeAccelerations();
            computePositions();
            computeVelocities();
            resolveCollisions();
        }

        public void printResults() {
            String fmt = "Body %d : % 8.6f  % 8.6f  % 8.6f | % 8.6f  % 8.6f  % 8.6f\n";
            for (int i = 0; i < bodies; ++i) {
                System.out.printf(
                    fmt,
                    i + 1,
                    positions[i].x, positions[i].y, positions[i].z,
                    velocities[i].x, velocities[i].y, velocities[i].z
                );
            }
        }
    }

    public static void main(String[] args) throws IOException {
        String filename = "nbody.txt";
        NBody nb = new NBody(filename);
        for (int i = 0; i < nb.timeSteps; ++i) {
            System.out.printf("\nCycle %s\n", i + 1);
            nb.simulate();
            nb.printResults();
        }
    }
}