code: private static double dot(double vector1[], double vector2[]) {
  
          double product = 0;
          assert vector1.length == vector2.length;
          for (int i = 0; i < vector1.length; i++) {
              product += vector1[i] * vector2[i];
          }
          return product;
  
      }
docs: returns the dot product of two vectors
  
  @since 1.8