public static Matrice multiply(Matrice a, Matrice b) throws MatrixException
      {  
          //Condition sur les dimensions
          if(a.columns != b.lines)
          {
            throw new MatrixException("Problème de dimension !");   
          }
            Matrice res = new Matrice(a.lines, b.columns);
            double temp = 0.0;
            int a_col = 0;

            for(int a_lines = 0; a_lines< a.lines; a_lines++)
            {
                for(int b_col = 0; b_col < b.columns; b_col++)
                {
                    for(a_col = 0; a_col < a.columns; a_col++)
                    {
                        temp += a.getValueAt(a_lines, a_col) * b.getValueAt(a_col, b_col);
                    }
                    res.setValueAt(a_lines, b_col, temp);
                    temp = 0.0;
                }
            }
          return res;
      }