Remove elements from collection while iterating

Question

AFAIK  there are two approaches    Iterate over a copy of the collection Use the iterator of the actual collection   For instance   List lt Foo gt  fooListCopy   new ArrayList lt Foo gt  fooList   for Foo foo   fooListCopy          modify actual fooList     and  Iterator lt Foo gt  itr   fooList iterator    while itr hasNext            modify actual fooList using itr remove       Are there any reasons to prefer one approach over the other  e g  preferring the first approach for the simple reason of readability

User · Answer

Old Timer Favorite  it still works   List lt String gt  list   for int i   list size   - 1  i  gt   0  --i             if list get i  contains  quot bad quot                              list remove i                Benefits   It only iterates over the list once No extra objects created  or other unneeded complexity No problems with trying to use the index of a removed item  because    well  think about it

User · Answer

Let me give a few examples with some alternatives to avoid a ConcurrentModificationException   Suppose we have the following collection of books  List lt Book gt  books   new ArrayList lt Book gt     books add new Book new ISBN  0-201-63361-2      books add new Book new ISBN  0-201-63361-3      books add new Book new ISBN  0-201-63361-4        Collect and Remove  The first technique consists in collecting all the objects that we want to delete  e g  using an enhanced for loop  and after we finish iterating  we remove all found objects   ISBN isbn   new ISBN  0-201-63361-2    List lt Book gt  found   new ArrayList lt Book gt     for Book book   books       if book getIsbn   equals isbn            found add book           books removeAll found     This is supposing that the operation you want to do is  delete     If you want to  add  this approach would also work  but I would assume you would iterate over a different collection to determine what elements you want to add to a second collection and then issue an addAll method at the end   Using ListIterator  If you are working with lists  another technique consists in using a ListIterator which has support for removal and addition of items during the iteration itself   ListIterator lt Book gt  iter   books listIterator    while iter hasNext         if iter next   getIsbn   equals isbn            iter remove              Again  I used the  remove  method in the example above which is what your question seemed to imply  but you may also use its add method to add new elements during iteration   Using JDK    8  For those working with Java 8 or superior versions  there are a couple of other techniques you could use to take advantage of it   You could use the new removeIf method in the Collection base class   ISBN other   new ISBN  0-201-63361-2    books removeIf b - gt  b getIsbn   equals other      Or use the new stream API   ISBN other   new ISBN  0-201-63361-2    List lt Book gt  filtered   books stream                               filter b - gt  b getIsbn   equals other                               collect Collectors toList       In this last case  to filter elements out of a collection  you reassign the original reference to the filtered collection  i e  books   filtered  or used the filtered collection to removeAll the found elements from the original collection  i e  books removeAll filtered     Use Sublist or Subset   There are other alternatives as well  If the list is sorted  and you want to remove consecutive elements you can create a sublist and then clear it   books subList 0 5  clear      Since the sublist is backed by the original list this would be an efficient way of removing this subcollection of elements   Something similar could be achieved with sorted sets using NavigableSet subSet method  or any of the slicing methods offered there   Considerations   What method you use might depend on what you are intending to do   The collect and removeAl technique works with any Collection  Collection  List  Set  etc    The ListIterator technique obviously only works with lists  provided that their given ListIterator implementation offers support for add and remove operations    The Iterator approach would work with any type of collection  but it only supports remove operations  With the ListIterator Iterator approach the obvious advantage is not having to copy anything since we remove as we iterate  So  this is very efficient   The JDK 8 streams example don t actually removed anything  but looked for the desired elements  and then we replaced the original collection reference with the new one  and let the old one be garbage collected  So  we iterate only once over the collection and that would be efficient  In the collect and removeAll approach the disadvantage is that we have to iterate twice  First we iterate in the foor-loop looking for an object that matches our removal criteria  and once we have found it  we ask to remove it from the original collection  which would imply a second iteration work to look for this item in order to remove it   I think it is worth mentioning that the remove method of the Iterator interface is marked as  optional  in Javadocs  which means that there could be Iterator implementations that throw UnsupportedOperationException if we invoke the remove method  As such  I d say this approach is less safe than others if we cannot guarantee the iterator support for removal of elements

User · Answer

Only second approach will work  You can modify collection during iteration using iterator remove   only  All other attempts will cause ConcurrentModificationException

User · Answer

In Java 8  there is another approach  Collection removeIf  eg   List lt Integer gt  list   new ArrayList lt  gt     list add 1   list add 2   list add 3    list removeIf i - gt  i  gt  2

User · Answer

Are there any reasons to prefer one approach over the other   The first approach will work  but has the obvious overhead of copying the list   The second approach will not work because many containers don t permit modification during iteration  This includes ArrayList   If the only modification is to remove the current element  you can make the second approach work by using itr remove    that is  use the iterator s remove   method  not the container s   This would be my preferred method for iterators that support remove

User · Answer

why not this   for  int i   0  i  lt  Foo size    i          if  Foo get i  equals  some test                Foo remove i            And if it s a map  not a list  you can use keyset

User · Answer

I would choose the second as you don t have to do a copy of the memory and the Iterator works faster  So you save memory and time

User · Answer

You can t do the second  because even if you use the remove   method on Iterator  you ll get an Exception thrown     Personally  I would prefer the first for all Collection instances  despite the additional overheard of creating the new Collection  I find it less prone to error during edit by other developers  On some Collection implementations  the Iterator remove   is supported  on other it isn t  You can read more in the docs for Iterator    The third alternative  is to create a new Collection  iterate over the original  and add all the members of the first Collection to the second Collection that are not up for deletion  Depending on the size of the Collection and the number of deletes  this could  significantly save on memory  when compared to the first approach

[java] Remove elements from collection while iterating

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