public class Vector4 {
  double data[] = new double[4];

  Vector4() {
    data[0] = data[1] = data[2] = data[3] = 0.0;
  }

  Vector4(double a, double b, double c, double d) {
    data[0] = a;
    data[1] = b;
    data[2] = c;
    data[3] = d;
  }

  public double get(int idx) { return data[idx]; }

  public double dotProduct(Vector4 v) {
    double sum = 0.0;
    for (int i=0; i<4; i++)
      sum += v.get(i)*this.get(i);

    return sum;
  }
}