chapter 18：type casting中的两个翻译难点，大家帮忙看看

[crazygx]

1. “Type casting is a way to check the type of an instance, and/or to treat that instance as if it is a different superclass or subclass from somewhere else in its own class hierarchy.”

主要是后半段。

1. 在downcasting中，optional form 和 forced form 这两个短语的翻译可能不准确。