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This code does not violate CON40CON06-J. Do not synchronize on a collection view if the backing collection is accessible, because while it does synchronize on a collection view (the synchronizedList), the backing collection is inaccessible, and therefore cannot be modified by any code.
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This compliant solution does not use Collections.synchronizedMap() because locking on the unsynchronized map provides sufficient thread-safety for this application. The guideline CON40CON06-J. Do not synchronize on a collection view if the backing collection is accessible provides more information about synchronizing on synchronizedMap objects.
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