How to copy Java Collections list

Question

I have an ArrayList and I want to copy it exactly  I use utility classes when possible on the assumption that someone spent some time making it correct  So naturally  I end up with the Collections class which contains a copy method   Suppose I have the following   List lt String gt  a   new ArrayList lt String gt     a add  a    a add  b    a add  c    List lt String gt  b   new ArrayList lt String gt  a size      Collections copy b a     This fails because basically it thinks b isn t big enough to hold a  Yes I know b has size 0  but it should be big enough now shouldn t it  If I have to fill b first  then Collections copy   becomes a completely useless function in my mind  So  except for programming a copy function  which I m going to do now  is there a proper way to do this

User · Answer

Copy isn t useless if you imagine the use case to copy some values into an existing collection  I e  you want to overwrite existing elements instead of inserting   An example  a    1 2 3 4 5  b    2 2 2 2 3 3 3 3 3 4 4 4   a copy b     1 2 3 4 5 3 3 3 3 4 4 4   However I d expect a copy method that would take additional parameters for the start index of the source and target collection  as well as a parameter for count   See Java BUG 6350752

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Why dont you just use addAll method       List a   new ArrayList             a add  1             a add  abc         List b   b addAll listA      b will be 1  abc   even if you have existing items in b or you want to pend some elements after it  such as   List a   new ArrayList         a add  1         a add  abc     List b   new ArrayList         b add  x         b addAll listA        b add  Y       b will be x  1  abc  Y

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private List lt Item gt  cloneItemList final List lt Item gt  items                Item   itemArray   new Item items size             itemArray   items toArray itemArray           return Arrays asList itemArray

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the simplest way to copy a List is to pass it to the constructor of the new list   List lt String gt  b   new ArrayList lt  gt  a     b will be a shallow copy of a  Looking at the source of Collections copy List List   I d never seen it before  it seems to be for coping the elements index by index  using List set int E  thus element 0 will over write element 0 in the target list etc etc  Not particularly clear from the javadocs I d have to admit   List lt String gt  a   new ArrayList lt  gt  a   a add  foo    b add  bar     List lt String gt  b   new ArrayList lt  gt  a      shallow copy  a      the following will all hold assert a get 0     b get 0   assert a get 1     b get 1   assert a equals b   assert a    b      a  is not the same object as  b

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Every other Object not --  you need to iterate and do a copy by yourself   To avoid this implement Cloneable   public class User implements Serializable  Cloneable        private static final long serialVersionUID   1L       private String user      private String password                Override     public Object clone             Object o   null          try             o   super clone              catch CloneNotSupportedException e                      return o                      public static void main String   args           List lt User gt  userList1   new ArrayList lt User gt            User user1   new User          user1 setUser  User1          user1 setPassword  pass1                     User user2   new User          user2 setUser  User2          user2 setPassword  pass2                     userList1  add user1         userList1  add user2          List lt User gt  userList2   new ArrayList lt User gt             for User u  userList1             u add  User u clone                      With this you can avoid                   for User u  userList1               User tmp   new User                tmp setUser u getUser               tmp setPassword u getPassword                               u add tmp

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The following output illustrates results of using copy constructor and Collections copy     Copy  1  2  3  to  1  2  3  using copy constructor   Copy  1  2  3  to  smaller   4  5  java lang IndexOutOfBoundsException  Source does not fit in dest         at java util Collections copy Collections java 556          at com farenda java CollectionsCopy copySourceToSmallerDest CollectionsCopy java 36          at com farenda java CollectionsCopy main CollectionsCopy java 14   Copy  1  2  to  same size   3  4  source   1  2  destination   1  2   Copy  1  2  to  bigger   3  4  5  source   1  2  destination   1  2  5   Copy  1  2  to  unmodifiable   4  5  java lang UnsupportedOperationException         at java util Collections UnmodifiableList set Collections java 1311          at java util Collections copy Collections java 561          at com farenda java CollectionsCopy copyToUnmodifiableDest CollectionsCopy java 68          at com farenda java CollectionsCopy main CollectionsCopy java 20    The source of full program is here  Java List copy  But the output is enough to see how java util Collections copy   behaves

User · Answer

Strings can be deep copied with  List lt String gt  b   new ArrayList lt String gt  a     because they are immutable  Every other Object not --  you need to iterate and do a copy by yourself

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Most answers here do not realize the problem  the user wants to have a COPY of the elements from first list to the second list  destination list elements are new objects and not reference to the elements of original list   means changing an element of second list should not change values for corresponding element of source list   For the mutable objects we cannot use ArrayList Collection  constructor because it will simple refer to the original list element and will not copy  You need to have a list cloner for each object when copying

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The answer by Stephen Katulka  accepted answer  is wrong  the second part   It explains that Collections copy b  a   does a deep copy  which it does not  Both  new ArrayList a   and Collections copy b  a   only do a shallow copy  The difference is  that the constructor allocates new memory  and copy      does not  which makes it suitable in cases where you can reuse arrays  as it has a performance advantage there   The Java standard API tries to discourage the use of deep copies  as it would be bad if new coders would use this on a regular basis  which may also be one of the reason why clone   is not public by default   The source code for Collections copy      can be seen on line 552 at  http   www java2s com Open-Source Java-Document 6 0-JDK-Core Collections-Jar-Zip-Logging-regex java util Collections java htm  If you need a deep copy  you have to iterate over the items manually  using a for loop and clone   on each object

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Just do   List a   new ArrayList     a add  a     a add  b     a add  c     List b   new ArrayList a     ArrayList has a constructor that will accept another Collection to copy the elements from

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To understand why Collections copy   throws an IndexOutOfBoundsException although you ve made the backing array of the destination list large enough  via the size   call on the sourceList   see the answer by Abhay Yadav in this related question  How to copy a java util List into another java util List

User · Answer

b has a capacity of 3  but a size of 0  The fact that ArrayList has some sort of buffer capacity is an implementation detail - it s not part of the List interface  so Collections copy List  List  doesn t use it  It would be ugly for it to special-case ArrayList   As MrWiggles has indicated  using the ArrayList constructor which takes a collection is the way to in the example provided   For more complicated scenarios  which may well include your real code   you may find the collections within Guava useful

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If you want to copy an ArrayList  copy it by using   List b   new ArrayList    b add  aa    b add  bb     List a   new ArrayList b

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With Java 8 being null-safe  you could use the following code   List lt String gt  b   Optional ofNullable a                            map list - gt   List lt String gt   new ArrayList lt  gt  list                             orElseGet Collections  emptyList     Or using a collector  List lt String gt  b   Optional ofNullable a                            map List  stream                            orElseGet Stream  empty                            collect Collectors toList

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And if you are using google guava  the one line solution would be   List lt String gt  b   Lists newArrayList a     This creates a mutable array list instance

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List b   new ArrayList a size      doesn t set the size  It sets the initial capacity  being how many elements it can fit in before it needs to resize    A simpler way of copying in this case is   List b   new ArrayList a

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As hoijui mentions  The selected answer from Stephen Katulka contains a comment about Collections copy that is incorrect  The author probably accepted it because the first line of code was doing the copy that he wanted  The additional call to Collections copy just copies again   Resulting in the copy happening twice    Here is code to prove it   public static void main String   args         List lt String gt  a   new ArrayList lt String gt         a add  a        a add  b        a add  c        List lt String gt  b   new ArrayList lt String gt  a        System out println  There should be no output after this line             Note  b is already a shallow copy of a      for  int i   0  i  lt  a size    i              if  a get i     b get i                 System out println  Oops  this was a deep copy        Note this is never called                          Now use Collections copy and note that b is still just a shallow copy of a     Collections copy b  a       for  int i   0  i  lt  a size    i              if  a get i     b get i                 System out println  Oops  i was wrong this was a deep copy       Note this is never called                          Now do a deep copy - requires you to explicitly copy each element     for  int i   0  i  lt  a size    i              b set i  new String a get i                   Now see that the elements are different in each      for  int i   0  i  lt  a size    i              if  a get i     b get i                 System out println  oops  i was wrong  a shallow copy was done        note this is never called

User · Answer

Calling  List lt String gt  b   new ArrayList lt String gt  a     creates a shallow copy of a within b  All elements will exist within b in the exact same order that they were within a  assuming it had an order    Similarly  calling     note  instantiating with a size   gives  b  enough capacity to hold everything List lt String gt  b   new ArrayList lt String gt  a size     Collections copy b  a     also creates a shallow copy of a within b  If the first parameter  b  does not have enough capacity  not size  to contain all of a s elements  then it will throw an IndexOutOfBoundsException  The expectation is that no allocations will be required by Collections copy to work  and if any are  then it throws that exception  It s an optimization to require the copied collection to be preallocated  b   but I generally do not think that the feature is worth it due to the required checks given the constructor-based alternatives like the one shown above that have no weird side effects   To create a deep copy  the List  via either mechanism  would have to have intricate knowledge of the underlying type  In the case of Strings  which are immutable in Java  and  NET for that matter   you don t even need a deep copy  In the case of MySpecialObject  you need to know how to make a deep copy of it and that is not a generic operation     Note  The originally accepted answer was the top result for Collections copy in Google  and it was flat out wrong as pointed out in the comments

[java] How to copy Java Collections list

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