How to copy a java util List into another java util List

Question

I have a List lt SomeBean gt  that is populated from a Web Service  I want to copy clone the contents of that list into an empty list of the same type  A Google search for copying a list suggested me to use Collections copy   method  In all the examples I saw  the destination list was supposed to contain the exact number of items for the copying to take place    As the list I am using is populated through a web service and it contains hundreds of objects  I cannot use the above technique  Or I am using it wrong     Anyways  to make it work  I tried to do something like this  but I still got an IndexOutOfBoundsException   List lt SomeBean gt  wsList   app allInOne template    List lt SomeBean gt  wsListCopy new ArrayList lt SomeBean gt  wsList size        Collections copy wsListCopy wsList   System out println wsListCopy size       I tried to use the wsListCopy wsList subList 0  wsList size    but I got a ConcurrentAccessException later in the code  Hit and trial      Anyways  my question is simple  how can I copy the entire content of my list into another List  Not through iteration  of course

User · Answer

originalArrayList addAll copyArrayofList    Please keep on mind whenever using the addAll   method for copy  the contents of both the array lists  originalArrayList and copyArrayofList  references to the same objects will be added to the list so if you modify any one of them then copyArrayofList also will also reflect the same change  If you don t want side effect then you need to copy each of element from the originalArrayList to the copyArrayofList  like using a for or while loop  for deep copy you can use below code snippet  but one more thing you need to do  implement the Cloneable interface and override the clone   method for SomeBean class  public static List lt SomeBean gt  cloneList List lt SomeBean gt  originalArrayList        List lt SomeBean gt  copyArrayofList   new ArrayList lt SomeBean gt  list size         for  SomeBean item   list  copyArrayofList add item clone         return clone

User · Answer

re  indexOutOfBoundsException  your sublist args are the problem  you need to end the sublist at size-1   Being zero-based  the last element of a list is always size-1  there is no element in the size position  hence the error

User · Answer

If you do not want changes in one list to effect another list try this It worked for me Hope this helps       public class MainClass     public static void main String   a         List list   new ArrayList        list add  A         List list2     List    ArrayList  list  clone          System out println list       System out println list2        list clear         System out println list       System out println list2           gt  Output      A     A          A

User · Answer

You can use addAll     eg   wsListCopy addAll wsList

User · Answer

Starting from Java 10   List lt E gt  oldList   List of    List lt E gt  newList   List copyOf oldList     List copyOf   returns an unmodifiable List containing the elements of the given Collection   The given Collection must not be null  and it must not contain any null elements   Also  if you want to create a deep copy of a List  you can find many good answers here

User · Answer

I tried to do something like this  but I still got an IndexOutOfBoundsException  I got a ConcurrentAccessException  This means you are modifying the list while you are trying to copy it  most likely in another thread   To fix this you have to either  use a collection which is designed for concurrent access   lock the collection appropriately so you can iterate over it  or allow you to call a method which does this for you   find a away to avoid needing to copy the original list

User · Answer

There is another method with Java 8 in a null-safe way  List lt SomeBean gt  wsListCopy   Optional ofNullable wsList       map Collection  stream       orElseGet Stream  empty       collect Collectors toList      If you want to skip one element  List lt SomeBean gt  wsListCopy   Optional ofNullable wsList       map Collection  stream       orElseGet Stream  empty       skip 1       collect Collectors toList      With Java 9   the stream method of Optional can be used Optional ofNullable wsList       stream        flatMap Collection  stream       collect Collectors toList

User · Answer

You should use the addAll method  It appends all of the elements in the specified collection to the end of the copy list  It will be a copy of your list  List lt String gt  myList   new ArrayList lt  gt     myList add  quot a quot    myList add  quot b quot    List lt String gt  copyList   new ArrayList lt  gt     copyList addAll myList

User · Answer

This is a really nice Java 8 way to do it   List lt String gt  list2   list1 stream   collect Collectors toList       Of course the advantage here is that you can filter and skip to only copy of part of the list   e g     don t copy the first element  List lt String gt  list2   list1 stream   skip 1  collect Collectors toList

User · Answer

subList function is a trick  the returned object is still in the original list  so if you do any operation in subList  it will cause the concurrent exception in your code  no matter it is single thread or multi thread

User · Answer

Just use this   List lt SomeBean gt  newList   new ArrayList lt SomeBean gt  otherList     Note  still not thread safe  if you modify otherList from another thread  then you may want to make that otherList  and even newList  a CopyOnWriteArrayList  for instance -- or use a lock primitive  such as ReentrantReadWriteLock to serialize read write access to whatever lists are concurrently accessed

User · Answer

I tried something similar and was able to reproduce the problem  IndexOutOfBoundsException   Below are my findings   1  The implementation of the Collections copy destList  sourceList  first checks the size of the destination list by calling the size   method  Since the call to the size   method will always return the number of elements in the list  0 in this case   the constructor ArrayList capacity  ensures only the initial capacity of the backing array and this does not have any relation to the size of the list  Hence we always get IndexOutOfBoundsException   2  A relatively simple way is to use the constructor that takes a collection as its argument   List lt SomeBean gt  wsListCopy new ArrayList lt SomeBean gt  wsList

User · Answer

I can t see any correct answer  If you want a deep copy you have to iterate and copy object manually  you could use a copy constructor

User · Answer

I was having the same problem ConcurrentAccessException and mysolution was to   List lt SomeBean gt  tempList   new ArrayList lt  gt      for  CartItem item   prodList      tempList add item     prodList clear    prodList   new ArrayList lt  gt  tempList     So it works only one operation at the time and avoids the Exeption

[java] How to copy a java.util.List into another java.util.List

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