Task: N-body problem
C++: #include <algorithm>
#include <iomanip>
#include <iostream>
#include <fstream>
#include <string>
#include <vector>

class Vector {
private:
    double px, py, pz;

public:
    Vector() : px(0.0), py(0.0), pz(0.0) {
        
    }

    Vector(double x, double y, double z) : px(x), py(y), pz(z) {
        
    }

    double mod() const {
        return sqrt(px*px + py * py + pz * pz);
    }

    Vector operator+(const Vector& rhs) const {
        return Vector(px + rhs.px, py + rhs.py, pz + rhs.pz);
    }

    Vector operator-(const Vector& rhs) const {
        return Vector(px - rhs.px, py - rhs.py, pz - rhs.pz);
    }

    Vector operator*(double s) const {
        return Vector(px*s, py*s, pz*s);
    }

    bool operator==(const Vector& rhs) const {
        return px == rhs.px
            && py == rhs.py
            && pz == rhs.pz;
    }

    friend std::istream& operator>>(std::istream&, Vector&);
    friend std::ostream& operator<<(std::ostream&, Vector&);
};

std::istream& operator>>(std::istream& in, Vector& v) {
    return in >> v.px >> v.py >> v.pz;
}

std::ostream& operator<<(std::ostream& out, Vector& v) {
    auto precision = out.precision();
    auto width = out.width();
    out << std::fixed << std::setw(width) << std::setprecision(precision) << v.px << "  ";
    out << std::fixed << std::setw(width) << std::setprecision(precision) << v.py << "  ";
    out << std::fixed << std::setw(width) << std::setprecision(precision) << v.pz;
    return out;
}

const Vector ORIGIN{ 0.0, 0.0, 0.0 };

class NBody {
private:
    double gc;
    int bodies;
    int timeSteps;
    std::vector<double> masses;
    std::vector<Vector> positions;
    std::vector<Vector> velocities;
    std::vector<Vector> accelerations;

    void resolveCollisions() {
        for (int i = 0; i < bodies; ++i) {
            for (int j = i + 1; j < bodies; ++j) {
                if (positions[i] == positions[j]) {
                    std::swap(velocities[i], velocities[j]);
                }
            }
        }
    }

    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] / pow((positions[i] - positions[j]).mod(), 3);
                    accelerations[i] = accelerations[i] + (positions[j] - positions[i]) * temp;
                }
            }
        }
    }

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

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

public:
    NBody(std::string& fileName) {
        using namespace std;

        ifstream ifs(fileName);
        if (!ifs.is_open()) {
            throw runtime_error("Could not open file.");
        }

        ifs >> gc >> bodies >> timeSteps;

        masses.resize(bodies);
        positions.resize(bodies);
        fill(positions.begin(), positions.end(), ORIGIN);
        velocities.resize(bodies);
        fill(velocities.begin(), velocities.end(), ORIGIN);
        accelerations.resize(bodies);
        fill(accelerations.begin(), accelerations.end(), ORIGIN);

        for (int i = 0; i < bodies; ++i) {
            ifs >> masses[i] >> positions[i] >> velocities[i];
        }

        cout << "Contents of " << fileName << '\n';
        cout << gc << ' ' << bodies << ' ' << timeSteps << '\n';
        for (int i = 0; i < bodies; ++i) {
            cout << masses[i] << '\n';
            cout << positions[i] << '\n';
            cout << velocities[i] << '\n';
        }
        cout << "\nBody   :      x          y          z    |     vx         vy         vz\n";
    }

    int getTimeSteps() {
        return timeSteps;
    }

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

    friend std::ostream& operator<<(std::ostream&, NBody&);
};

std::ostream& operator<<(std::ostream& out, NBody& nb) {
    for (int i = 0; i < nb.bodies; ++i) {
        out << "Body " << i + 1 << " : ";
        out << std::setprecision(6) << std::setw(9) << nb.positions[i];
        out << " | ";
        out << std::setprecision(6) << std::setw(9) << nb.velocities[i];
        out << '\n';
    }
    return out;
}

int main() {
    std::string fileName = "nbody.txt";
    NBody nb(fileName);

    for (int i = 0; i < nb.getTimeSteps(); ++i) {
        std::cout << "\nCycle " << i + 1 << '\n';
        nb.simulate();
        std::cout << nb;
    }

    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();
        }
    }
}