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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 CON40-J. Do not synchronize on a collection view if the backing collection is accessible provides more information about synchronizing on synchronizedMap objects.
The integer overflow check that should be used before incrementing has been omitted for brevity. To prevent overflow, the caller must ensure that the increment() method is called no more than Integer.MAX_VALUE times for any key. Refer to INT00-J. Perform explicit range checking to ensure integer operations do not overflow for more information.
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