How do I copy the contents of one ArrayList into another

Question

I have some data structures  and I would like to use one as a temporary  and another as not temporary   ArrayList lt Object gt  myObject   new ArrayList lt Object gt     ArrayList lt Object gt  myTempObject   new ArrayList lt Object gt         fill myTempObject here         make myObject contain the same values as myTempObject myObject   myTempObject     free up memory by clearing myTempObject myTempObject clear      now the problem with this of course is that myObject is really just pointing to myTempObject  and so once myTempObject is cleared  so is myObject   How do I retain the values from myTempObject in myObject using java

User · Answer

Suppose you have two arraylist of String type . Like

ArrayList<String> firstArrayList ;//This array list is not having any data.

ArrayList<String> secondArrayList = new ArrayList<>();//Having some data.

Now we have to copy the data of second array to first arraylist like this,

firstArrayList = new ArrayList<>(secondArrayList );

Done!!

User · Answer

originalArrayList addAll copyArrayList    Please Note  When using the addAll   method to copy  the contents of both the array lists  originalArrayList and copyArrayList  refer to the same objects or contents  So if you modify any one of them the other will also reflect the same change   If you don t wan t this then you need to copy each element from the originalArrayList to the copyArrayList  like using a for or while loop

User · Answer

Copy of one list into second is quite simple   you can do that as below -   ArrayList lt List1 gt  list1  new ArrayList lt  gt     ArrayList lt List1 gt  list2  new ArrayList lt  gt       this will your copy your list1 into list2 list2 addAll list1

User · Answer

Here is a workaround to copy all the objects from one arrayList to another    ArrayList lt Object gt  myObject   new ArrayList lt Object gt     ArrayList lt Object gt  myTempObject   new ArrayList lt Object gt      myObject addAll myTempObject subList 0  myTempObject size        subList is intended to return a List with a range of data  so you can copy the whole arrayList or part of it

User · Answer

Supopose you want to copy oldList into a new ArrayList object called newList  ArrayList lt Object gt  newList   new ArrayList lt  gt       for  int i   0   i lt oldList size   i         newList add oldList get i         These two lists are indepedant  changes to one are not reflected to the other one

User · Answer

You need to clone   the individual object  Constructor and other methods perform shallow copy  You may try  Collections copy method

User · Answer

There are no implicit copies made in java via the assignment operator  Variables contain a reference value  pointer  and when you use   you re only coping that value   In order to preserve the contents of myTempObject you would need to make a copy of it   This can be done by creating a new ArrayList using the constructor that takes another ArrayList   ArrayList lt Object gt  myObject   new ArrayList lt Object gt  myTempObject     Edit  As Bohemian points out in the comments below  is this what you re asking  By doing the above  both ArrayLists  myTempObject and myObject  would contain references to the same objects  If you actually want a new list that contains new copies of the objects contained in myTempObject then you would need to make a copy of each individual object in the original ArrayList

User · Answer

Straightforward way to make deep copy of original list is to add all element from one list to another list   ArrayList lt Object gt  originalList   new ArrayList lt Object gt     ArrayList lt Object gt  duplicateList   new ArrayList lt Object gt      for Object o   originalList        duplicateList add o       Now If you make any changes to originalList it will not impact duplicateList

User · Answer

Lets try the example       ArrayList lt String gt  firstArrayList   new ArrayList lt  gt                 firstArrayList add  One                firstArrayList add  Two                firstArrayList add  Three                firstArrayList add  Four                firstArrayList add  Five                firstArrayList add  Six                  copy array list content into another array list              ArrayList lt String gt  secondArrayList new ArrayList lt  gt                 secondArrayList addAll firstArrayList                 print all the content of array list             Iterator itr   secondArrayList iterator                while  itr hasNext                      System out println itr next                     In print output as below  One Two Three Four Five Six  We can also do by using clone   method for which is used to create exact copy  for that try you can try as like     ArrayList lt String gt secondArrayList    ArrayList lt String gt   firstArrayList clone          And then print by using iterator         Iterator itr   secondArrayList iterator                    while  itr hasNext                          System out println itr next

User · Answer

Came across this while facing the same issue myself   Saying arraylist1   arraylist2 sets them both to point at the same place so if you alter either the data alters and thus both lists always stay the same   To copy values into an independent list I just used foreach to copy the contents   ArrayList list1   new ArrayList    ArrayList list2   new ArrayList      fill list1 in whatever way you currently are   foreach  lt type gt  obj in list1        list2 Add obj

User · Answer

to copy one list into the other list  u can use the method called Collection copy myObject myTempObject  now after executing these line of code u can see all the list values in the myObject

User · Answer

You can use such trick   myObject   new ArrayList lt Object gt  myTempObject     or use   myObject    ArrayList lt Object gt  myTempObject clone      You can get some information about clone   method here  But you should remember  that all these ways will give you a copy of your List  not all of its elements  So if you change one of the elements in your copied List  it will also be changed in your original List

[java] How do I copy the contents of one ArrayList into another?

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