All of the above can be conditional upon the operating system type and revision, time of day, arbitrary user-defined classes, presence or absence of files, directories, or data in files, and so on.
上述所有这些根据操作系统类型和修订版本、一天中的时间、任意用户定义的类、文件中文件、目录或数据的有无等等可以是有条件的。
Expect to see few (if any) user defined classes fall into this category.
期望越少(如果有的话)的用户定义类落入此种境地。
System classes, for instance, are not modifiable, while user-defined interface classes can be modified, but can't include actual implementation code.
例如，系统类是不可修改的，而用户定义接口类虽然可以修改，但是不能包含实际的实现代码。
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