How to avoid ConcurrentModificationException while removing elements from ArrayList while iterating it

Question

I m trying to remove some elements from an ArrayList while iterating it like this   for  String str   myArrayList        if  someCondition            myArrayList remove str             Of course  I get a ConcurrentModificationException when trying to remove items from the list at the same time when iterating myArrayList  Is there some simple solution to solve this problem

User · Answer

List myArrayList    Collections synchronizedList new ArrayList        add your elements    myArrayList add     myArrayList add     myArrayList add     synchronized myArrayList        Iterator i   myArrayList iterator          while  i hasNext              Object  object   i next

User · Answer

As an alternative to everyone else s answers I ve always done something like this   List lt String gt  toRemove   new ArrayList lt String gt     for  String str   myArrayList        if  someCondition            toRemove add str           myArrayList removeAll toRemove     This will avoid you having to deal with the iterator directly  but requires another list   I ve always preferred this route for whatever reason

User · Answer

Java 8 user can do that  list removeIf           List lt String gt  list   new ArrayList lt  gt  Arrays asList  a    b    c         list removeIf e - gt   someCondition      It will remove elements in the list  for which someCondition is satisfied

User · Answer

You can use the iterator remove   function to remove the object from underlying collection object  But in this case you can remove the same object and not any other object from the list   from here

User · Answer

No  no  NO   In single threated tasks you don t need to use Iterator  moreover  CopyOnWriteArrayList  due to performance hit    Solution is much simpler  try to use canonical for loop instead of for-each loop   According to Java copyright owners  some years ago Sun  now Oracle  for-each loop guide  it uses iterator to walk through collection and just hides it to make code looks better  But  unfortunately as we can see  it produced more problems than profits  otherwise this topic would not arise   For example  this code will lead to java util ConcurrentModificationException when entering next iteration on modified ArrayList              process collection         for  SomeClass currElement  testList                 SomeClass founDuplicate   findDuplicates currElement               if  founDuplicate    null                    uniqueTestList add founDuplicate                   testList remove testList indexOf currElement                              But following code works just fine          process collection     for  int i   0  i  lt  testList size    i              SomeClass currElement   testList get i            SomeClass founDuplicate   findDuplicates currElement           if  founDuplicate    null                uniqueTestList add founDuplicate               testList remove testList indexOf currElement                i--    to avoid skipping of shifted element                   So  try to use indexing approach for iterating over collections and avoid for-each loop  as they are not equivalent  For-each loop uses some internal iterators  which check collection modification and throw ConcurrentModificationException exception  To confirm this  take a closer look at the printed stack trace when using first example that I ve posted   Exception in thread  main  java util ConcurrentModificationException     at java util AbstractList Itr checkForComodification AbstractList java 372      at java util AbstractList Itr next AbstractList java 343      at TestFail main TestFail java 43    For multithreading use corresponding multitask approaches  like synchronized keyword

User · Answer

While other suggested solutions work  If you really want the solution to be made thread safe you should replace ArrayList with CopyOnWriteArrayList        List lt String gt  s   new ArrayList lt  gt       Will throw exception     List lt String gt  s   new CopyOnWriteArrayList lt  gt         s add  B        Iterator lt String gt  it   s iterator        s add  A           Below removes only  B  from List     while  it hasNext              s remove it next               System out println s

User · Answer

One alternative method is convert your List to array  iterate them and remove them directly from the List based on your logic   List lt String gt  myList   new ArrayList lt String gt        You can use either list or set  myList add  abc    myList add  abcd    myList add  abcde    myList add  abcdef    myList add  abcdefg     Object   obj   myList toArray     for Object o obj         if condition          myList remove o toString

User · Answer

Use an Iterator and call remove     Iterator lt String gt  iter   myArrayList iterator     while  iter hasNext          String str   iter next         if  someCondition          iter remove

User · Answer

You have to use the iterator s remove   method  which means no enhanced for loop   for  final Iterator iterator   myArrayList iterator    iterator hasNext            iterator next        if  someCondition            iterator remove

User · Answer

If you want to modify your List during traversal  then you need to use the Iterator  And then you can use iterator remove   to remove the elements during traversal

[java] How to avoid "ConcurrentModificationException" while removing elements from `ArrayList` while iterating it?

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