ConcurrentModificationException for ArrayList

Question

I have the following piece of code   private String toString List lt DrugStrength gt  aDrugStrengthList        StringBuilder str   new StringBuilder            for  DrugStrength aDrugStrength   aDrugStrengthList                if   aDrugStrength isValidDrugDescription                      aDrugStrengthList remove aDrugStrength                                   str append aDrugStrengthList           if  str indexOf         -1                str insert str lastIndexOf         n                             return str toString        When I try to run it  I get ConcurrentModificationException  can anyone explain why it happens  even if the code is running in same thread  And how could I avoid it

User · Answer

While iterating through the loop  you are trying to change the List value in the remove   operation  This will result in ConcurrentModificationException   Follow the below code  which will achieve what you want and yet will not throw any exceptions  private String toString List aDrugStrengthList            StringBuilder str   new StringBuilder        List removalList   new ArrayList        for  DrugStrength aDrugStrength   aDrugStrengthList            if   aDrugStrength isValidDrugDescription                  removalList add aDrugStrength                       aDrugStrengthList removeAll removalList       str append aDrugStrengthList       if  str indexOf         -1            str insert str lastIndexOf         n                         return str toString

User · Answer

We can use concurrent collection classes to avoid ConcurrentModificationException while iterating over a collection  for example CopyOnWriteArrayList instead of ArrayList   Check this post for ConcurrentHashMap   http   www journaldev com 122 hashmap-vs-concurrenthashmap- E2 80 93-example-and-exploring-iterator

User · Answer

You can t remove from list if you re browsing it with  for each  loop  You can use Iterator  Replace   for  DrugStrength aDrugStrength   aDrugStrengthList        if   aDrugStrength isValidDrugDescription              aDrugStrengthList remove aDrugStrength             With   for  Iterator lt DrugStrength gt  it   aDrugStrengthList iterator    it hasNext            DrugStrength aDrugStrength   it next        if   aDrugStrength isValidDrugDescription              it remove

User · Answer

I like a reverse order for loop such as   int size   list size    for  int i   size - 1  i  gt   0  i--        if remove           list remove i             because it doesn t require learning any new data structures or classes

User · Answer

there should has a concurrent implemention of List interface supporting such operation   try java util concurrent CopyOnWriteArrayList class

User · Answer

Like the other answers say  you can t remove an item from a collection you re iterating over  You can get around this by explicitly using an Iterator and removing the item there   Iterator lt Item gt  iter   list iterator    while iter hasNext        Item blah   iter next      if            iter remove       Removes the  current  item

[java] ConcurrentModificationException for ArrayList

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