清单3给出了包含目录和文件信息的一对类的代码。
Listing 3 gives the code for the pair of classes holding directory and file information.
一旦一个类已经被装载，它们总是作为一对存在。
Once a class has been properly loaded, these will always exist as a pair.
同一对象类的对象定义存储在同一个对象存储库文件中。
Object definitions of the same object class are stored in the same object repository file.
在这两个类间添加一个一对一的关联关系，如图4所示。
Add a One-to-One Association relationship between these two classes, as shown in Figure 4.
每个一对一的关联在表中任一类中充当外键。
Each one-to-one association is buried as a foreign key in the table of either class.
一对，一双：同一种类的两件；一对。
一对，一双：同一种类的两件；一对。
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