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There is no need to copy immutable sub-objects. Also, avoid using the sub-object's {{clone()}} method because it can be overridden when the sub-object's class is non-final. Moreover, it produces only a shallow copy. The sub-objects ({{date}}) themselves must be non-final so that defensive copying can occur. It is also inadvisable to use the {{writeUnshared()}} and {{readUnshared()}} methods as an alternative \[[Bloch 082008|AA. Java References#Bloch 08]\].

\[[API 062006|AA. Java References#API 06]\]
\[[Sun 062006|AA. Java References#Sun 06]\] "Serialization specification: A.6  Guarding Unshared Deserialized Objects"
\[[Bloch 082008|AA. Java References#Bloch 08]\] Item 76: "Write readObject methods defensively"