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This map (as with the majority of the maps, collections and bags in commons-collections) should be externally synchronized (by using - for example - Collections.synchronizeMap) when used in a multithreaded environment. When not appropriately synchronized, our analysis of the code indicates that NullPointerExceptions may be thrown. However, all those who have analysed the code have yet to find a use case when where a NullPointerException will may be thrown when the map is appropriately synchronized.
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