Why am I not getting a java util ConcurrentModificationException in this example

Question

Note  I am aware of the Iterator remove   method   In the following code sample  I don t understand why the List remove in main method throws ConcurrentModificationException  but not in the remove method   public class RemoveListElementDemo           private static final List lt Integer gt  integerList       static           integerList   new ArrayList lt Integer gt             integerList add 1           integerList add 2           integerList add 3              public static void remove Integer toRemove            for Integer integer   integerList                if integer equals toRemove                                     integerList remove integer                                      public static void main String    args                            remove Integer valueOf 2             Integer toRemove   Integer valueOf 3           for Integer integer   integerList                if integer equals toRemove                                     integerList remove integer

User · Answer

In my case I did it like this   int cursor   0  do       if  integer equals remove           integerList remove cursor       else cursor      while  cursor    integerList size

User · Answer

Check your code man      In the main method you are trying to remove the 4th element which is not there and hence the error  In the remove   method you are trying to remove the 3rd element which is there and hence no error

User · Answer

Change Iterator for each into for loop to solve     And the Reason is      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    --Referred Java Docs

User · Answer

This snippet will always throw a ConcurrentModificationException   The rule is  You may not modify  add or remove elements from the list  while iterating over it using an Iterator  which happens when you use a for-each loop     JavaDocs   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   Hence if you want to modify the list  or any collection in general   use iterator  because then it is aware of the modifications and hence those will be handled properly    Hope this helps

User · Answer

If you use copy-on-write collections it will work  however when you use list iterator    the returned Iterator will always reference the collection of elements as it was when   as below    list iterator   was called  even if another thread modifies the collection  Any mutating methods called on a copy-on-write   based Iterator or ListIterator  such as add  set  or remove  will throw an UnsupportedOperationException   import java util List  import java util concurrent CopyOnWriteArrayList   public class RemoveListElementDemo           private static final List lt Integer gt  integerList       static           integerList   new CopyOnWriteArrayList lt  gt             integerList add 1           integerList add 2           integerList add 3              public static void remove Integer remove            for Integer integer   integerList                if integer equals remove                                     integerList remove integer                                      public static void main String    args                            remove Integer valueOf 2             Integer remove   Integer valueOf 3           for Integer integer   integerList                if integer equals remove                                     integerList remove integer

User · Answer

Here s why  As it is says in the Javadoc      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    This check is done in the next   method of the iterator  as you can see by the stacktrace   But we will reach the next   method only if hasNext   delivered true  which is what is called by the for each to check if the boundary is met  In your remove method  when hasNext   checks if it needs to return another element  it will see that it returned two elements  and now after one element was removed the list only contains two elements  So all is peachy and we are done with iterating  The check for concurrent modifications does not occur  as this is done in the next   method which is never called   Next we get to the second loop  After we remove the second number the hasNext method will check again if can return more values  It has returned two values already  but the list now only contains one  But the code here is   public boolean hasNext             return cursor    size        1    2  so we continue to the next   method  which now realizes that someone has been messing with the list and fires the exception   Hope that clears your question up   Summary  List remove   will not throw ConcurrentModificationException when it removes the second last element from the list

User · Answer

The forward iterator method does not work when removing items   You can remove the element without error  but you will get a runtime error when you try to access removed items    You can t use the iterator because as pushy shows it will cause a ConcurrentModificationException  so use a regular for loop instead  but step backwards through it     List lt Integer gt  integerList  integerList   new ArrayList lt Integer gt     integerList add 1   integerList add 2   integerList add 3    int size  integerList size       Item to remove Integer remove   Integer valueOf 3     A solution   Traverse the array in reverse order if you are going to remove a list element   Simply by going backwards through the list you avoid visiting an item that has been removed  which removes the exception     To remove items from the list  start from the end and go backwards through the arrayList   This way if we remove one from the beginning as we go through  then we will avoid getting a runtime error   for java lang IndexOutOfBoundsException or java util ConcurrentModificationException as when we used the iterator for  int i size-1  i gt  -1  i--        if  integerList get i  equals remove              integerList remove i

User · Answer

One way to handle it it to remove something from a copy of a Collection  not Collection itself   if applicable  Clone the original collection it to make a copy via a Constructor      This exception may be thrown by methods that have detected concurrent   modification of an object when such modification is not permissible    For your specific case  first off  i don t think final is a way to go considering you intend to modify the list past declaration  private static final List lt Integer gt  integerList    Also consider modifying a copy instead of the original list   List lt Integer gt  copy   new ArrayList lt Integer gt  integerList    for Integer integer   integerList        if integer equals remove                             copy remove integer

User · Answer

This runs fine on Java 1 6      javac RemoveListElementDemo java     java RemoveListElementDemo     cat RemoveListElementDemo java    import java util    public class RemoveListElementDemo           private static final List lt Integer gt  integerList       static           integerList   new ArrayList lt Integer gt             integerList add 1           integerList add 2           integerList add 3              public static void remove Integer remove            for Integer integer   integerList                if integer equals remove                                     integerList remove integer                                      public static void main String    args                            remove Integer valueOf 2             Integer remove   Integer valueOf 3           for Integer integer   integerList                if integer equals remove                                     integerList remove integer

User · Answer

I had that same problem but in case that I was adding en element into iterated list  I made it this way  public static void remove Integer remove        for int i 0  i lt integerList size    i                here is maybe fine to deal with integerList get i   null         if integerList get i  equals remove                                 integerList remove i                       Now everything goes fine because you don t create any iterator over your list  you iterate over it  manually   And condition i  lt  integerList size   will never fool you because when you remove add something into List size of the List decrement increment    Hope it helps  for me that was solution

[java] Why am I not getting a java.util.ConcurrentModificationException in this example?

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