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Note that the keys of a hash table do not need to be deep copied; 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() functions. Mutable objects whose equals() or hashCode() method results may be modified are not suitable keys.
This example is also more amenable to efficient garbage collection, as described in rule "OBJ11-J. Write garbage-collection-friendly code."
Exceptions
OBJ05-EX0: When callers expose only unmodifiable views of an object to a method, the method may freely use the unmodifiable 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 unmodifiable views. (See rule "SEC14-J. Provide sensitive mutable classes with unmodifiable wrappers.")
Risk Assessment
Returning references to internal object state (mutable or immutable) can render an application susceptible to information leaks and corruption of its objects' states, which, consequently, violates class invariants. Control flow can also be affected in some cases.
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<ac:structured-macro ac:name="unmigrated-wiki-markup" ac:schema-version="1" ac:macro-id="1fad17da838c3182-c7be5bdf-4574403d-b0f69afa-fa74b6a57ff538b1fba98828"><ac:plain-text-body><![CDATA[ | [[Goetz 2006 | AA. Bibliography#Goetz 06]] | 3.2. Publication and Escape: Allowing Internal Mutable State to Escape | ]]></ac:plain-text-body></ac:structured-macro> | |
<ac:structured-macro ac:name="unmigrated-wiki-markup" ac:schema-version="1" ac:macro-id="7d52209eba63ff20-5520ddca-4cb64680-91a88d2b-50c267b7decb4d3c8db8a05c"><ac:plain-text-body><![CDATA[ | [[Gong 2003 | AA. Bibliography#Gong 03]] | 9.4 Private Object State and Object Immutability | ]]></ac:plain-text-body></ac:structured-macro> | |
<ac:structured-macro ac:name="unmigrated-wiki-markup" ac:schema-version="1" ac:macro-id="6d7f84af5a8b3961-ef2e080f-4c514a9a-9c5981b2-2a26dfd2f19a508154602e1a"><ac:plain-text-body><![CDATA[ | [[Haggar 2000 | AA. Bibliography#Haggar 00]] | [Practical Java Praxis 64: Use clone for Immutable Objects When Passing or Receiving Object References to Mutable Objects | http://www.informit.com/articles/article.aspx?p=20530] | ]]></ac:plain-text-body></ac:structured-macro> |
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