How to add element in List while iterating in java

Question

Say I have a List like   List lt String gt  list   new ArrayList lt  gt     list add  a    list add  h    list add  f    list add  s      While iterating through this list I want to add an element at the end of the list  But I don t want to iterate through the newly added elements that is I want to iterate up to the initial size of the list   for  String s   list          Here I want to add new element if needed while iterating      Can anybody suggest me how can I do this

User · Answer

You could iterate on a copy  clone  of your original list   List lt String gt  copy   new ArrayList lt String gt  list   for  String s   copy           And if you have to add an element to the list  add it to the original one      list add  some element        Note that it is not even possible to add a new element to a list while iterating on it  because it will result in a ConcurrentModificationException

User · Answer

Just iterate the old-fashion way  because you need explicit index handling   List myList           int length   myList size    for int i   0  i  lt  length  i         String s   myList get i         add items here  if you want to

User · Answer

I do this by adding the elements to an new  empty tmp List  then adding the tmp list to the original list using addAll     This prevents unnecessarily copying a large source list     Imagine what happens when the OP s original list has a few million items in it  for a while you ll suck down twice the memory   In addition to conserving resources  this technique also prevents us from having to resort to 80s-style for loops and using what are effectively array indexes which could be unattractive in some cases

User · Answer

You can t use a foreach statement for that  The foreach is using internally an iterator      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     From ArrayList javadoc   In the foreach statement you don t have access to the iterator s add method and in any case that s still not the type of add that you want because it does not append at the end  You ll need to traverse the list manually   int listSize   list size    for int i   0  i  lt  listSize    i    list add  whatever      Note that this is only efficient for Lists that allow random access  You can check for this feature by checking whether the list implements the RandomAccess marker interface  An ArrayList has random access  A linked list does not

User · Answer

To help with this I created a function to make this more easy to achieve it  public static  lt T gt  void forEachCurrent List lt T gt  list  Consumer lt T gt  action        final int size   list size        for  int i   0  i  lt  size  i              action accept list get i             Example     List lt String gt  l   new ArrayList lt  gt         l add  quot 1 quot        l add  quot 2 quot        l add  quot 3 quot        forEachCurrent l  e - gt            l add e    quot A quot            l add e    quot B quot            l add e    quot C quot                l forEach System out  println

User · Answer

Iterate through a copy of the list and add new elements to the original list    for  String s   new ArrayList lt String gt  list              list add  u        See How to make a copy of ArrayList object which is type of List

[java] How to add element in List while iterating in java?

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