How to avoid java util ConcurrentModificationException when iterating through and removing elements from an ArrayList

Question

I have an ArrayList that I want to iterate over  While iterating over it I have to remove elements at the same time  Obviously this throws a java util ConcurrentModificationException   What is the best practice to handle this problem  Should I clone the list first   I remove the elements not in the loop itself but another part of the code   My code looks like this   public class Test         private ArrayList lt A gt  abc   new ArrayList lt A gt          public void doStuff             for  A a   abc           a doSomething               public void removeA A a            abc remove a             a doSomething might call Test removeA

User · Answer

Do somehting simple like this   for  Object object   ArrayList lt String gt   list clone          list remove object

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Do the loop in the normal way  the java util ConcurrentModificationException is an error related to the elements that are accessed   So try   for int i   0  i  lt  list size    i         lista get i  action

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You are trying to remove value from list in advanced  for loop   which is not possible  even if you apply any trick  which you did in your code   Better way is to code iterator level as other advised here   I wonder how people have not suggested traditional for loop approach   for  int i   0  i  lt  lStringList size    i           String lValue   lStringList get  i        if lValue equals   Not Required                   lStringList remove lValue            i--               This works as well

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You can also use CopyOnWriteArrayList instead of an ArrayList  This is the latest recommended approach by from JDK 1 5 onwards

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An alternative Java 8 solution using stream           theList   theList stream                filter element - gt   shouldBeRemoved element                collect Collectors toList       In Java 7 you can use Guava instead           theList   FluentIterable from theList               filter new Predicate lt String gt                       Override                 public boolean apply String element                        return  shouldBeRemoved element                                                 toImmutableList      Note  that the Guava example results in an immutable list which may or may not be what you want

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ERROR There was a mistake when I added to the same list from where I took elements  fun  lt T gt  MutableList lt T gt  mathList  fun   T  - gt  T   MutableList lt T gt        for  i in this            this add  fun i      lt ---   ERROR           return this    lt --- ERROR    DECISION Works great when adding to a new list  fun  lt T gt  MutableList lt T gt  mathList  fun   T  - gt  T   MutableList lt T gt        val newList   mutableListOf lt T gt       lt ---   DECISION     for  i in this            newList add  fun i      lt ---   DECISION           return newList    lt ---   DECISION

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Just add a break after your ArrayList remove A  statement

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While iterating the list  if you want to remove the element is possible  Let see below my examples        ArrayList lt String gt   names   new ArrayList lt String gt             names add  abc            names add  def            names add  ghi            names add  xyz      I have the above names of Array list  And i want to remove the  def  name from the above list    for String name   names       if name equals  def             names remove  def              The above code throws the ConcurrentModificationException exception because you are modifying the list while iterating      So  to remove the  def  name  from Arraylist by doing this way   Iterator lt String gt  itr   names iterator                while itr hasNext         String name   itr next        if name equals  def             itr remove              The above code  through iterator we can remove the  def  name from the Arraylist and try to print the array  you would be see the below output   Output     abc  ghi  xyz

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Instead of using For each loop  use normal for loop  for example the below code removes all the element in the array list without giving java util ConcurrentModificationException  You can modify the condition in the loop according to your use case  for int i 0  i lt abc size    i              e remove i

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Should I clone the list first    That will be the easiest solution  remove from the clone  and copy the clone back after removal   An example from my rummikub game   SuppressWarnings  unchecked   public void removeStones       ArrayList lt Stone gt  clone    ArrayList lt Stone gt   stones clone         remove the stones moved to the table   for  Stone stone   stones          if  stone isOnTable               clone remove stone                 stones    ArrayList lt Stone gt   clone clone      sortStones

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Here is an example where I use a different list to add the objects for removal  then afterwards I use stream foreach to remove elements from original list    private ObservableList lt CustomerTableEntry gt  customersTableViewItems   FXCollections observableArrayList        private void removeOutdatedRowsElementsFromCustomerView         ObjectProperty lt TimeStamp gt  currentTimestamp   new SimpleObjectProperty lt  gt  TimeStamp getCurrentTime         long diff      long diffSeconds      List lt Object gt  objectsToRemove   new ArrayList lt  gt         for CustomerTableEntry item  customersTableViewItems            diff   currentTimestamp getValue   getTime   - item timestamp getValue   getTime            diffSeconds   diff   1000   60          if diffSeconds  gt  10                   Element has been idle for too long  meaning no communication  hence remove it             System out printf  - Idle element   s  - will be removed n   item getUserName                 objectsToRemove add item                       objectsToRemove stream   forEach o - gt  customersTableViewItems remove o

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You should really just iterate back the array in the traditional way  Every time you remove an element from the list  the elements after will be push forward  As long as you don t change elements other than the iterating one  the following code should work   public class Test        private ArrayList lt A gt  abc   new ArrayList lt A gt          public void doStuff            for int i    abc size   - 1   i  gt   0  i--               abc get i  doSomething               public void removeA A a           abc remove a

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I arrive late I know but I answer this because I think this solution is simple and elegant   List lt String gt  listFixed   new ArrayList lt String gt     List lt String gt  dynamicList   new ArrayList lt String gt      public void fillingList         listFixed add  Andrea        listFixed add  Susana        listFixed add  Oscar        listFixed add  Valeria        listFixed add  Kathy        listFixed add  Laura        listFixed add  Ana        listFixed add  Becker        listFixed add  Abraham        dynamicList addAll listFixed      public void updatingListFixed         for  String newList   dynamicList            if   listFixed contains newList                 listFixed add newList                          this is for add elements if you want eraser also       String removeRegister         for  String fixedList   listFixed            if   dynamicList contains fixedList                 removeResgister   fixedList                      fixedList remove removeRegister       All this is for updating from one list to other and you can make all from just one list and in method updating you check both list and can eraser or add elements betwen list  This means both list always it same size

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Two options    Create a list of values you wish to remove  adding to that list within the loop  then call  originalList removeAll valuesToRemove  at the end Use the remove   method on the iterator itself  Note that this means you can t use the enhanced for loop    As an example of the second option  removing any strings with a length greater than 5 from a list   List lt String gt  list   new ArrayList lt String gt         for  Iterator lt String gt  iterator   list iterator    iterator hasNext            String value   iterator next        if  value length    gt  5            iterator remove

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Use Iterator instead of Array List  Have a set be converted to iterator with type match  And move to the next element and remove  Iterator lt Insured gt  itr   insuredSet iterator    while  itr hasNext           itr next        itr remove        Moving to the next is important here as it should take the index to remove element

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If your goal is to remove all elements from the list  you can iterate over each item  and then call   list clear

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In Java 8 you can use the Collection Interface and do this by calling the removeIf method   yourList removeIf  A a  - gt  a value    2     More information can be found here

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What about of   import java util Collections   List lt A gt  abc   Collections synchronizedList new ArrayList lt  gt

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In my case  the accepted answer is not working  It stops Exception but it causes some inconsistency in my List  The following solution is perfectly working for me   List lt String gt  list   new ArrayList lt  gt     List lt String gt  itemsToRemove   new ArrayList lt  gt      for  String value  list       if  value length    gt  5       your condition        itemsToRemove add value          list removeAll itemsToRemove     In this code  I have added the items to remove  in another list and then used list removeAll method to remove all required items

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One option is to modify the removeA method to this -  public void removeA A a Iterator lt A gt  iterator         iterator remove a            But this would mean your doSomething   should be able to pass the iterator to the remove method  Not a very good idea   Can you do this in two step approach     In the first loop when you iterate over the list   instead of removing the selected elements   mark them as to be deleted  For this   you may simply copy these elements   shallow copy   into another List   Then   once your iteration is done   simply do a removeAll from the first list all elements in the second list

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From the JavaDocs of the ArrayList     The iterators returned by this class s iterator and listIterator   methods are fail-fast  if  the list is structurally modified at any   time after the iterator is created  in any way except  through the   iterator s own remove or add methods  the iterator will throw a    ConcurrentModificationException

[java] How to avoid java.util.ConcurrentModificationException when iterating through and removing elements from an ArrayList

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