In each subsystem, the paper analyses its form, working process, user's model and typical algorithms from the pointofobject-oriented and technology of active mechanism separately.
The goal of the data Access Object pattern is to provide a single pointof contact to a particular data source.
数据访问对象模式的目的是提供到特定数据源的单个联系点。
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Note that the path must be a closed loop - that is, the first pointof the first Edge object in the path must have the same location as the last pointof the last Edge object.