Other instances in which this pattern is useful include logging and printing, such as when you want only one instance of a class to access and control one particular printer at a time.
在其他实例中，此模式也是有用的，包括登录和打印，例如，当您想要一个能够每次访问并控制一个特别打印机的类实例。
The ServiceUI class provides a single method to display the printer selection dialog
ServiceUI 类提供一个单独的方法来显示打印机选择对话框
Imagine that the name printer, complete with its synchronization protocol, is your concurrent class.
设想NamePrinter以及它的同步协议是并行类。
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