In Java can you modify a List while iterating through it

Question

I understand that in Java a Collection lt E gt  should not be modified while iterating through it  such as removing or adding elements  But what about changing the elements in a List  For example  what if we have  List lt String gt  letters   new ArrayList lt String gt     letters add  A    letters add  B    letters add  C    int i   0   for  String letter   letters        letters set i   D        i        So  I m not talking about modifying the object stored at an element  I m talking about changing what the object is  The size of the List is not being changed  but the object at the index is changing  so technically the List is being modified  My boss claims this code is fine  and it does appear to work   but I still don t think it is correct  Would a different way of doing it  maybe using the set E e  method of a ListIterator  be better

User · Answer

Use Java 8 s removeIf     To remove safely   letters removeIf x - gt   x equals  A

User · Answer

Use CopyOnWriteArrayList and if you want to remove it  do the following   for  Iterator lt String gt  it   userList iterator    it hasNext            if  wordsToRemove contains word                 it remove

User · Answer

Java 8 s stream   interface provides a great way to update a list in place   To safely update items in the list  use map     List lt String gt  letters   new ArrayList lt  gt         add stuff to list  letters   letters stream   map x - gt   D   collect Collectors toList        To safely remove items in place  use filter      letters stream   filter x - gt   x equals  A    collect Collectors toList

User · Answer

There is nothing wrong with the idea of modifying an element inside a list while traversing it  don t modify the list itself  that s not recommended   but it can be better expressed like this   for  int i   0  i  lt  letters size    i          letters set i   D        At the end the whole list will have the letter  D  as its content  It s not a good idea to use an enhanced for loop in this case  you re not using the iteration variable for anything  and besides you can t modify the list s contents using the iteration variable   Notice that the above snippet is not modifying the list s structure - meaning  no elements are added or removed and the lists  size remains constant  Simply replacing one element by another doesn t count as a structural modification  Here s the link to the documentation quoted by  ZouZou in the comments  it states that      A structural modification is any operation that adds or deletes one or more elements  or explicitly resizes the backing array  merely setting the value of an element is not a structural modification

[java] In Java, can you modify a List while iterating through it?

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