Calling remove in foreach loop in Java

Question

In Java  is it legal to call remove on a collection when iterating through the collection using a foreach loop   For instance   List lt String gt  names        for  String name   names          Do something    names remove name       As an addendum  is it legal to remove items that have not been iterated over yet  For instance      Assume that the names list as duplicate entries List lt String gt  names        for  String name   names           Do something     while  names remove name

User · Answer

The java design of the  enhanced for loop  was to not expose the iterator to code  but the only way to safely remove an item is to access the iterator  So in this case you have to do it old school    for Iterator lt String gt  i   names iterator    i hasNext              String name   i next             Do Something        i remove         If in the real code the enhanced for loop is really worth it  then you could add the items to a temporary collection and call removeAll on the list after the loop   EDIT  re addendum   No  changing the list in any way outside the iterator remove   method while iterating will cause problems  The only way around this is to use a CopyOnWriteArrayList  but that is really intended for concurrency issues   The cheapest  in terms of lines of code  way to remove duplicates is to dump the list into a LinkedHashSet  and then back into a List if you need   This preserves insertion order while removing duplicates

User · Answer

It s better to use an Iterator when you want to remove element from a list  because the source code of remove is   if  numMoved  gt  0      System arraycopy elementData  index 1  elementData  index               numMoved   elementData --size    null    so  if you remove an element from the list  the list will be restructure  the other element s index will be changed  this can result something that you want to happened

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Those saying that you can t safely remove an item from a collection except through the Iterator aren t quite correct  you can do it safely using one of the concurrent collections such as ConcurrentHashMap

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Try this 2  and change the condition to  WINTER  and you will wonder    public static void main String   args      Season add  Fr  hling      Season add  Sommer      Season add  Herbst      Season add  WINTER      for  String s   Season       if  s equals  Sommer          System out println s       continue          Season remove  Fr  hling

User · Answer

for  String name   new ArrayList lt String gt  names            Do something     names remove nameToRemove       You clone the list names and iterate through the clone while you remove from the original list  A bit cleaner than the top answer

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Make sure this is not code smell   Is it possible to reverse the logic and be  inclusive  rather than  exclusive    List lt String gt  names        List lt String gt  reducedNames        for  String name   names          Do something    if  conditionToIncludeMet         reducedNames add name     return reducedNames    The situation that led me to this page involved old code that looped through a List using indecies to remove elements from the List  I wanted to refactor it to use the foreach style     It looped through an entire list of elements to verify which ones the user had permission to access  and removed the ones that didn t have permission from the list   List lt Service gt  services       for  int i 0  i lt services size    i          if   isServicePermitted user  services get i             services remove i       To reverse this and not use the remove   List lt Service gt  services       List lt Service gt  permittedServices       for  Service service services        if  isServicePermitted user  service            permittedServices add service     return permittedServices    When would  remove  be preferred  One consideration is if gien a large list or expensive  add   combined with only a few removed compared to the list size   It might be more efficient to only do a few removes rather than a great many adds   But in my case the situation did not merit such an optimization

User · Answer

I didn t know about iterators  however here s what I was doing until today to remove elements from a list inside a loop    List lt String gt  names         for  i names size  -1 i gt  0 i--               Do something         names remove i        This is always working  and could be used in other languages or structs not supporting iterators

User · Answer

You don t want to do that  It can cause undefined behavior depending on the collection  You want to use an Iterator directly  Although the for each construct is syntactic sugar and is really using an iterator  it hides it from your code so you can t access it to call Iterator remove      The behavior of an iterator is   unspecified if the underlying   collection is modified while the   iteration is in progress in any way   other than by calling this method    Instead write your code   List lt String gt  names        Iterator lt String gt  it   names iterator    while  it hasNext           String name   it next           Do something     it remove        Note that the code calls Iterator remove  not List remove   Addendum   Even if you are removing an element that has not been iterated over yet  you still don t want to modify the collection and then use the Iterator  It might modify the collection in a way that is surprising and affects future operations on the Iterator

User · Answer

Yes you can use the for-each loop  To do that you have to maintain a separate list to hold removing items and then remove that list from names list using removeAll   method   List lt String gt  names            introduce a separate list to hold removing items List lt String gt  toRemove  new ArrayList lt String gt      for  String name   names          Do something  perform conditional checks    toRemove add name         names removeAll toRemove       now names list holds expected values

User · Answer

Use   remove   of Interator or  Use   CopyOnWriteArrayList

User · Answer

To safely remove from a collection while iterating over it you should use an Iterator   For example   List lt String gt  names        Iterator lt String gt  i   names iterator    while  i hasNext         String s   i next       must be called before you can call i remove         Do something    i remove        From the Java Documentation       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  Thus  in the face of concurrent   modification  the iterator fails quickly and cleanly  rather than   risking arbitrary  non-deterministic behavior at an undetermined time   in the future    Perhaps what is unclear to many novices is the fact that iterating over a list using the for foreach constructs implicitly creates an iterator which is necessarily inaccessible  This info can be found here

[java] Calling remove in foreach loop in Java

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