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Note that making deep copies of the keys of a hash table is unnecessary; shallow copying of the references suffices because a hash table's contract dictates that its keys must produce consistent results to the equals() and hashCode() methods. Mutable objects whose equals() or hashCode() method results may be modified are not suitable keys.
Exceptions
OBJ05-J-EX0: When a method is called with only an immutable view of an object, that method may freely use the immutable view without defensive copying. This decision should be made early in the design of the API. Note that new callers of such methods must also expose only immutable views.
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