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If the hash table contained references to mutable data such as a series of Date objects, every one of those objects must be copied by using a copy constructor or method. For further details, refer to FIO31FIO00-J. Defensively copy mutable inputs and mutable internal components and OBJ10-J. Provide mutable classes with a clone method to allow passing instances to untrusted code safely. Note that the keys of a hash table need not be deep copied; shallow copying of the references suffices because a hash table's contract dictates that it cannot hold duplicate keys.
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