the​ ​calculator​ ​class​ ​will​ ​instantiate​ ​an​ ​object​ ​of​ ​the​ ​converter​ ​class​ ​in​ ​order​ ​to have​ ​the​ ​infix​ ​expression​ ​converted​ ​to​ ​a​ ​postfix​ ​expression.