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<Integer> a = new ArrayList<Integer>();         // added 0-9 to ArrayList                   for(int i=0;i<9;i++)             a.add(new Integer(i));          // initialize the Iterator         Iterator<Integer> 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.

Answer 1

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.

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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<Foo> fooList = new ArrayList<Foo>();     for (int i = 0; i < 9; i++)       fooList.add(new Foo(i, i));      // Standard iterator sufficient for altering elements     Iterator<Foo> 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<Foo> 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<?> list) {     Iterator<?> 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]