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Causes : The Collection is internally modified, while a thread is iterating over it. OR after the creation of an iterator, the Collection is internally modified by any method other than the iterator’s own methods for removal and addition.
In order to avoid more than one threads accessing or modifying the same object, you can synchronize them over the object, in order to allow only one thread to manipulate it over time.
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at java.util.ArrayList$Itr.checkForComodification(Unknown Source)[?:1.8.0_25]
at java.util.ArrayList$Itr.remove(Unknown Source)[?:1.8.0_25]