是类引用,可以引用类:下面是帮助中的一段:
Sometimes operations are performed on a class itself, rather than on instances
of a class (that is, objects). This happens, for example, when you call a
constructor method using a class reference. You can always refer to a specific
class using its name, but at times it is necessary to declare variables or
parameters that take classes as values, and in these situations you need
class-reference types.
