How to concat two ArrayLists

Question

I have two ArrayLists of equal size  List 1 consists of 10 names and list 2 consists of their phone numbers    I want to concat the names and number into one ArrayList  How do I do this

User · Answer

If you want to do it one line and you do not want to change list1 or list2 you can do it using stream  List lt String gt  list1   Arrays asList  London    Paris    List lt String gt  list2   Arrays asList  Moscow    Tver     List lt String gt  list   Stream concat list1 stream   list2 stream    collect Collectors toList

User · Answer

You can use  addAll   to add the elements of the second list to the first   array1 addAll array2     Edit  Based on your clarification above   i want a single String in the new Arraylist which has both name and number     you would want to loop through the first list and append the item from the second list to it   Something like this   int length   array1 size    if  length    array2 size         Too many names  or too many numbers        Fail   ArrayList lt String gt  array3   new ArrayList lt String gt  length      Make a new list for  int i   0  i  lt  length  i         Loop through every name phone number combo     array3 add array1 get i          array2 get i       Concat the two  and add it     If you put in   array1     a    b    c   array2     1    2    3     You will get   array3     a 1    b 2    c 3

User · Answer

ArrayList lt String gt  resultList   new ArrayList lt String gt     resultList addAll arrayList1   resultList addAll arrayList2

User · Answer

One ArrayList1 add to data   mArrayList1 add data     and Second ArrayList2 to add other data   mArrayList2 addAll mArrayList1

User · Answer

var arr3   new arraylist    for int i 0  j 0  k 0  i lt arr1 size   arr2 size    i         if i amp 1          arr3 add arr1 j          else         arr3 add arr2 k          as you say   the names and numbers beside each other

User · Answer

add one ArrayList to second ArrayList as   Arraylist1 addAll Arraylist2     EDIT   if you want to Create new ArrayList from two existing ArrayList then do as   ArrayList lt String gt  arraylist3 new ArrayList lt String gt      arraylist3 addAll Arraylist1      add first arraylist  arraylist3 addAll Arraylist2      add Second arraylist

User · Answer

for a lightweight list that does not copy the entries  you may use sth like this   List lt Object gt  mergedList   new ConcatList lt  gt  list1  list2     here the implementation   public class ConcatList lt E gt  extends AbstractList lt E gt         private final List lt E gt  list1      private final List lt E gt  list2       public ConcatList final List lt E gt  list1  final List lt E gt  list2            this list1   list1          this list2   list2              Override     public E get final int index            return getList index  get getListIndex index                Override     public E set final int index  final E element            return getList index  set getListIndex index   element               Override     public void add final int index  final E element            getList index  add getListIndex index   element               Override     public E remove final int index            return getList index  remove getListIndex index                Override     public int size             return list1 size     list2 size                Override     public void clear             list1 clear            list2 clear               private int getListIndex final int index            final int size1   list1 size            return index  gt   size1   index - size1   index             private List lt E gt  getList final int index            return index  gt   list1 size     list2   list1

[java] How to concat two ArrayLists?

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