13.1. Data Modeling

Although JDO does not prescribe any specific technical requirements for Java classes to be made persistent, such as the presence of certain methods or a common superclass, and most arbitrary Java classes can be made persistent using JDO, certain best practices for modeling persistent classes do exist.[1]

[1] As explained earlier, it is not entirely correct to state that all classes can be made persistent; certain system-type classes such as System, Thread, Socket, and File can in fact not be JDO persistence-capable. Most user-defined classes can be however, and so for most practical applications, this subtle point can be ignored. (One would rarely hold references to or subclass a Thread or similar class in a data object class.) ...

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