code: protected double evalG(DoubleSolution solution) {
      double g = 0.0;
      for (int i = 1; i < solution.getNumberOfVariables(); i++) {
        g += solution.getVariableValue(i);
      }
      double constant = 9.0 / (solution.getNumberOfVariables() - 1);
  
      return constant * g + 1.0;
    }
docs: Returns the value of the ZDT1 function G.
  
  @param solution Solution