Library Puzzlers - Java Puzzlers: Traps, Pitfalls, and Corner Cases - page 181

ensures that the second occurrence of the string literal "Mickey" in our program refers to the same
String instance as the first, so our use of an IdentityHashMap in place of a general-purpose Map
implementation, such as HashMap , does not affect the program's behavior. Our naive analysis
neglects two details, but these details effectively cancel each other out.
The important lesson of this puzzle is: Don't use IdentityHashMap unless you need its identity-
based semantics; it is not a general-purpose Map implementation. These semantics are useful for
implementing topology-preserving object graph transformations , such as serialization or deep-
copying. A secondary lesson is that string constants are interned. As mentioned in Puzzle 13 ,
programs should rarely, if ever, depend on this behavior for their correct operation.
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