How to change value of ArrayList element in java

Question

Please help me with below code   I get the same output even after changing the value  import java util     class Test       public static void main String   args            ArrayList lt Integer gt  a   new ArrayList lt Integer gt                added 0-9 to ArrayList                   for int i 0 i lt 9 i                a add new Integer i                initialize the Iterator         Iterator lt Integer gt  i   a iterator                changed the value of first element in List         if i hasNext                  Integer x   i next                x   Integer valueOf 9                         initialized the iterator again and print all the elements         i   a iterator            while i hasNext                System out print i next                    Output   012345678   Value 9 is not updating

User · Answer

I agree with Duncan    I have tried it with mutable object but still get the same problem    I got a simple solution to this    use ListIterator instead Iterator and use set method of ListIterator  ListIterator lt Integer gt  i   a listIterator      changed the value of first element in List Integer x  null          if i hasNext                  x   i next                x   Integer valueOf 9                   set method sets the recent iterated element in ArrayList     i set x             initialized the iterator again and print all the elements         i   a listIterator            while i hasNext            System out print i next       But this constraints me to use this only for ArrayList only which can use ListIterator   i will have same problem with any other Collection

User · Answer

I think the problem is that you think the statement       x   Integer valueOf 9         causes that the value of  9  get  stored  into    the Object on which x is referencing   But thats wrong   Instead the statement causes something similar as if you would call  x   new Integer 9      If you have a look to the java source code  you will see what happens in Detail   Here is the code of the  valueOf int i   method in the  Integer  class   public static Integer valueOf int i        assert IntegerCache high  gt   127      if  i  gt   IntegerCache low  amp  amp  i  lt   IntegerCache high          return IntegerCache cache i    -IntegerCache low        return new Integer i       and further  whenever the IntegerCache class is used for the first time the following script gets invoked   static              high value may be configured by property         int h   127          String integerCacheHighPropValue               sun misc VM getSavedProperty  java lang Integer IntegerCache high            if  integerCacheHighPropValue    null                int i   parseInt integerCacheHighPropValue               i   Math max i  127                  Maximum array size is Integer MAX VALUE             h   Math min i  Integer MAX VALUE -  -low  -1                     high   h           cache   new Integer  high - low    1           int j   low          for int k   0  k  lt  cache length  k                cache k    new Integer j             You see that either a new Integer Object is created with  new Integer i   in the valueOf method         or a reference to a Integer Object which is stored in the IntegerCache is returned   In both cases x will reference to a new Object   And this is why the reference to the Object in your list get lost when you call      x   Integer valueOf 9     Instead of doing so  in combination with a ListIterator use      i set Integer valueOf 9          after you got the element you want to change with      i next

User · Answer

You re trying to change the value in the list  but all you re doing is changing the reference of x  Doing the following only changes x  not anything in the collection   x   Integer valueOf 9     Additionally  Integer is immutable  meaning you can t change the value inside the Integer object  which would require a different method to do anyway   This means you need to replace the whole object  There is no way to do this with an Iterator  without adding your own layer of boxing   Do the following instead   a set 0  9

User · Answer

The list is maintaining an object reference to the original value stored in the list  So when you execute this line   Integer x   i next      Both x and the list are storing a reference to the same object  However  when you execute   x   Integer valueOf 9     nothing has changed in the list  but x is now storing a reference to a different object  The list has not changed  You need to use some of the list manipulation methods  such as   list set index  Integer valueof 9     Note  this has nothing to do with the immutability of Integer  as others are suggesting  This is just basic Java object reference behaviour     Here s a complete example  to help explain the point  Note that this makes use of the ListIterator class  which supports removing setting items mid-iteration   import java util     public class ListExample      public static void main String   args         List lt Foo gt  fooList   new ArrayList lt Foo gt         for  int i   0  i  lt  9  i          fooList add new Foo i  i            Standard iterator sufficient for altering elements     Iterator lt Foo gt  iterator   fooList iterator         if  iterator hasNext            Foo foo   iterator next          foo x   99        foo y   42             printList fooList               List iterator needed for replacing elements     ListIterator lt Foo gt  listIterator   fooList listIterator         if  listIterator hasNext               Need to call next  before set        listIterator next             Replace item returned from next         listIterator set new Foo 99 99               printList fooList          private static void printList List lt   gt  list        Iterator lt   gt  iterator   list iterator        while  iterator hasNext            System out print iterator next               System out println           private static class Foo       int x      int y       Foo int x  int y          this x   x        this y   y              Override     public String toString           return String format    d   d    x  y                 This will print    99  42  1  1  2  2  3  3  4  4  5  5  6  6  7  7  8  8   99  99  1  1  2  2  3  3  4  4  5  5  6  6  7  7  8  8

User · Answer

Change it to for int i 0 i lt  9 i

User · Answer

Use the set method to replace the old value with a new one   list set  2   New

User · Answer

Where you say you re changing the value of the first element  x   Integer valueOf 9    You re changing x to point to an entirely new Integer  but never using it again  You re not changing the collection in any way  Since you re working with ArrayList  you can use ListIterator if you want an iterator that allows you to change the elements  this is the snippet of your code that would need to be changed     initialize the Iterator ListIterator lt Integer gt  i   a listIterator      changed the value of frist element in List if i hasNext              i next            i set Integer valueOf 9              Change the element the iterator is currently at      New iterator  and print all the elements Iterator iter   a iterator    while iter hasNext            System out print iter next      gt  gt  912345678  Sadly the same cannot be extended to other collections like Set lt T gt   Implementation details  a HashSet for example being implemented as a hash table and changing the object could change the hash value and therefore the iteration order  makes Set lt T gt  a  quot add new remove only quot  type of data structure  and changing the content at all while iterating over it is not safe

[java] How to change value of ArrayList element in java

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