Java ArrayList copy

Question

I have an ArrayList l1 of size 10  I assign l1 to new list reference type l2  Will l1 and l2 point to same ArrayList object  Or is a copy of the ArrayList object assigned to l2   When using the l2 reference  if I update the list object  it reflects the changes in the l1 reference type also   For example    List lt Integer gt  l1   new ArrayList lt Integer gt     for  int i   1  i  lt   10  i          l1 add i      List l2   l1  l2 clear      Is there no other way to assign a copy of a list object to a new reference variable  apart from creating 2 list objects  and doing copy on collections from old to new

User · Answer

Java doesn t pass objects  it passes references  pointers  to objects  So yes  l2 and l1 are two pointers to the same object   You have to make an explicit copy if you need two different list with the same contents

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Just for completion  All the answers above are going for a shallow copy - keeping the reference of the original objects  I you want a deep copy  your  reference-  class in the list have to implement a clone   copy method  which provides a deep copy of a single object  Then you can use  newList addAll oldList stream   map s- gt s clone    collect Collectors toList

User · Answer

List copyOf   unmodifiable list You asked   Is there no other way to assign a copy of a list  Java 9 brought the List of methods for using literals to create an unmodifiable List of unknown concrete class  LocalDate today   LocalDate now  ZoneId of   quot Africa Tunis quot        List lt  LocalDate  gt  dates   List of       today minusDays  1         Yesterday     today                      Today     today plusDays  1          Tomorrow     Along with that we also got List copyOf  This method too returns an unmodifiable List of unknown concrete class  List lt  String  gt  colors   new ArrayList lt  gt   4                 Creates a modifiable  List    colors add    quot AliceBlue quot      colors add    quot PapayaWhip quot      colors add    quot Chartreuse quot      colors add    quot DarkSlateGray quot      List lt  String  gt  masterColors   List copyOf  colors          Creates an unmodifiable  List    By    unmodifiable    we mean the number of elements in the list  and the object referent held in each slot as an element  is fixed  You cannot add  drop  or replace elements  But the object referent held in each element may or may not be mutable  colors remove  2                 SUCCEEDS   masterColors remove  2           FAIL - ERROR   See this code run live at IdeOne com   dates toString     2020-02-02  2020-02-03  2020-02-04  colors toString     AliceBlue  PapayaWhip  DarkSlateGray  masterColors toString     AliceBlue  PapayaWhip  Chartreuse  DarkSlateGray   You asked about object references  As others said  if you create one list and assign it to two reference variables  pointers   you still have only one list  Both point to the same list  If you use either pointer to modify the list  both pointers will later see the changes  as there is only one list in memory  So you need to make a copy of the list  If you want that copy to be unmodifiable  use the List copyOf method as discussed in this Answer  In this approach  you end up with two separate lists  each with elements that hold a reference to the same content objects  For example  in our example above using String objects to represent colors  the color objects are floating around in memory somewhere  The two lists hold pointers to the same color objects  Here is a diagram   The first list colors is modifiable  This means that some elements could be removed as seen in code above  where we removed the original 3rd element Chartreuse  index of 2   ordinal 3   And elements can be added  And the elements can be changed to point to some other String such as OliveDrab or CornflowerBlue  In contrast  the four elements of masterColors are fixed  No removing  no adding  and no substituting another color  That List implementation is unmodifiable

User · Answer

Yes l1 and l2 will point to the same reference  same object   If you want to create a new ArrayList based on the other ArrayList you do this   List lt String gt  l1   new ArrayList lt String gt     l1 add  Hello    l1 add  World    List lt String gt  l2   new ArrayList lt String gt  l1     A new arrayList  l2 add  Everybody      The result will be l1 will still have 2 elements and l2 will have 3 elements

User · Answer

There is a method addAll   which will serve the purpose of copying One ArrayList to another   For example you have two Array Lists  sourceList and targetList  use below code   targetList addAll sourceList

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Another convenient way to copy the values from src ArrayList to dest Arraylist is as follows   ArrayList lt String gt  src   new ArrayList lt String gt     src add  test string1    src add  test string2    ArrayList lt String gt  dest  new ArrayList lt String gt     dest addAll src     This is actual copying of values and not just copying of reference

User · Answer

Try to use Collections copy destination  source

User · Answer

Yes  assignment will just copy the value of l1  which is a reference  to l2  They will both refer to the same object   Creating a shallow copy is pretty easy though   List lt Integer gt  newList   new ArrayList lt  gt  oldList      Just as one example

[java] Java ArrayList copy

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