关于thinkphp5.1中的服务容器

interface Father{ 
        public function write()
}
class Son implements Father{
        public function write()
        {
             echo '儿子写字';
        }
}
class Daughter implements Father{
        public function write()
        {
            echo '女儿写字';
        }
}

class Index{
        public function son()
        {
          bind('son', 'Son');//第二个参数是son类的命名空间
          $son = app('son');
          var_dump($son->write());//打印出'儿子写字'
        }
        public function daughter()
        {
          bind('daughter', 'Daughter');//第二个参数是daughter类的命名空间
          $daughter= app('daughter');
          var_dump($daughter->write());//打印出'女儿写字'
        }
}