tokens: Without seeing more code , I 'm assuming that another thread is probably modifying the contents of someList , in which case you 'll need to implement some sort of synchronization policy , such as a synchronized block when you attempt to extract a sub-list from it - - that 's me shooting in the dark .
ner_tags: O O O O O O O O O O O O O O O O O B-Variable O O O O O O O O O O O O O O O O O O B-Function O O O O O O O B-Data_Structure O O O O O O O O O O O O