How to use Iterator in Java
By: Fazal
Before you can access a collection through an iterator, you must obtain one. Each of the collection classes provides an iterator( ) method that returns an iterator to the start of the collection. By using this iterator object, you can access each element in the collection, one element at a time. In general, to use an iterator to cycle through the contents of a collection, follow these steps:
- Obtain an iterator to the start of the collection by calling the collection's iterator( )
- Set up a loop that makes a call to hasNext( ). Have the loop iterate as long as hasNext( ) returns true.
- Within the loop, obtain each element by calling next( ).
For collections that implement List, you can also obtain an iterator by calling ListIterator. As explained, a list iterator gives you the ability to access the collection in either the forward or backward direction and lets you modify an element. Otherwise, ListIterator is used just like Iterator.
Here is an example that implements these steps, demonstrating both Iterator and ListIterator. It uses an ArrayList object, but the general principles apply to any type of collection. Of course, ListIterator is available only to those collections that implement the List interface.
// Demonstrate iterators.
import java.util.*;
class IteratorDemo {
public static void main(String args[]) {
// create an array list
ArrayList al = new ArrayList();
// add elements to the array list
al.add("C");
al.add("A");
al.add("E");
al.add("B");
al.add("D");
al.add("F");
// use iterator to display contents of al
System.out.print("Original contents of al: ");
Iterator itr = al.iterator();
while(itr.hasNext()) {
Object element = itr.next();
System.out.print(element + " ");
}
System.out.println();
// modify objects being iterated
ListIterator litr = al.listIterator();
while(litr.hasNext()) {
Object element = litr.next();
litr.set(element + "+");
}
System.out.print("Modified contents of al: ");
itr = al.iterator();
while(itr.hasNext()) {
Object element = itr.next();
System.out.print(element + " ");
}
System.out.println();
// now, display the list backwards
System.out.print("Modified list backwards: ");
while(litr.hasPrevious()) {
Object element = litr.previous();
System.out.print(element + " ");
}
System.out.println();
}
}
The output is shown here:
Original contents of al: C A E B D F
Modified contents of al: C+ A+ E+ B+ D+ F+
Modified list backwards: F+ D+ B+ E+ A+ C+
Pay special attention to how the list is displayed in reverse. After the list is modified, litr points to the end of the list. (Remember, litr.hasNext( ) returns false when the end of the list has been reached.) To traverse the list in reverse, the program continues to use litr, but this time it checks to see whether it
Archived Comments
1. I see you don't monetize your site, don't waste your
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View Tutorial By: 86Ronda at 2017-07-27 06:29:44
2. Do you mind if I quote a couple of your articles as long as I provide
credit and sources bac
View Tutorial By: Lap dance Barcelona at 2017-07-02 19:24:20
3. Nice one.your example proves that if you are apply list iterator on collection object than one can c
View Tutorial By: Nisarg pathak at 2015-09-30 17:23:37
4. Very nice tutorial demonstrating iteration of array list. While i was iterating Set elements. Thanx
View Tutorial By: akshay at 2013-10-08 08:43:40
5. Very nice tutorial demonstrating iteration of array list. While i was iterating Set elements. Thanx
View Tutorial By: akshay at 2013-10-08 08:42:09
6. Thanks i had problem with this util module.
View Tutorial By: james thompson at 2013-09-08 22:25:50
7. tanx...
View Tutorial By: sevda at 2013-06-30 21:48:38
8. explanation is good but i have one doubt what is the use of Object in the program
View Tutorial By: sulthan at 2012-10-09 06:13:14
9. I am able to iterate the list.Now i want to print value on screen.How to do this?
View Tutorial By: akanksha at 2012-09-26 05:25:12
10. Very usefull info.. Thank alot
View Tutorial By: Srikanth at 2012-09-04 15:44:30
11. Very good program to understand a very tough matter like Iterator.
View Tutorial By: Arin at 2012-08-08 06:50:42
12. Thanks a Lot for gud explaination
View Tutorial By: Mahi at 2012-06-23 06:01:51
13. Thanks for giving list iterator example.
it was nice and once again thank u
View Tutorial By: sandeepika at 2012-05-04 15:57:42
14. List<string> al = new List<string>() { "C", "A", "E", &quo
View Tutorial By: .net whit LinQ gonna change your life.. at 2012-04-23 15:10:55
15. superb superb maza aggaya maza aggaya
View Tutorial By: pramodh at 2012-04-16 05:21:54
16. Many thanks for your nice explanation.
View Tutorial By: Zahra at 2012-04-10 06:03:56
17. It is so clear. Thanks for this material.
View Tutorial By: John Ortiz at 2012-02-26 01:48:32
18. You've explained it excelent
Tanks a lot
View Tutorial By: Mich at 2012-01-18 16:05:34
19. I would like to know how to obtain the object that the Iterator is actually referencing. Not all Ar
View Tutorial By: Steve at 2012-01-18 15:58:31
20. good explanation,
Thanks
View Tutorial By: venkatesh at 2012-01-04 08:43:48
21. thanks excellent explanation!!!!!!!!
View Tutorial By: divya at 2012-01-01 17:37:32
22. Good Explanation of code. But object declaration inside while loop is not perfect. People may confus
View Tutorial By: Nitesh at 2011-12-28 12:37:09
23. good tutorial
View Tutorial By: mohang at 2011-12-24 05:24:56
24. its really a good post... very informative..
View Tutorial By: javagava at 2011-12-13 19:49:08
25. Nice tutorial , just to add <a href="http://javarevisited.blogspot.com/2011/10/java-iterator
View Tutorial By: Rajiv at 2011-12-01 08:43:32
26. Thanks dude....
Good work...
View Tutorial By: ram at 2011-11-18 12:05:31
27. Thanks, short and sweet answer!
View Tutorial By: A_College_Student at 2011-11-16 03:45:20
28. good explanation ,it is very very understandable
View Tutorial By: chaitanya at 2011-10-27 16:21:55
29. good job man, its all i needed to understand iterators
View Tutorial By: m3mph1s at 2011-10-11 05:01:17
30. Really Clear
View Tutorial By: Yasar Arafath at 2011-09-08 07:38:42
31. The sample code was all I needed to figure out how to use iterators. Thanks.
View Tutorial By: David at 2011-08-28 07:34:28
32. good explanation for the java beginners...could you explain about the hasNext(),hasPrevious() method
View Tutorial By: karthik at 2011-08-19 11:17:53
33. This is Nice start for programmer '
Thanks ..
View Tutorial By: Sagar Ikhankar at 2011-07-22 06:12:44
34. Also note that for reverse ordering, If you have a Collection that implements either Deque (like Lin
View Tutorial By: Dwight at 2011-07-13 22:13:45
35. Nice Example.Good work.keep it up.
Thanks,
-Lucky
View Tutorial By: Lucky at 2011-07-08 10:36:38
36. good and nice
View Tutorial By: Elayaraja j at 2011-06-18 05:30:07
37. extremely good
View Tutorial By: rahul at 2011-06-09 05:59:43
38. I am still going through the tutorial, but it seems to trail off or that last sentence is truncated.
View Tutorial By: shadowwraith at 2011-06-08 08:00:57
39. Great stuff, learned more from this example than reading java books on the same topic for hours.
View Tutorial By: David Bandelin at 2011-03-22 07:10:38
40. its really good.
View Tutorial By: raj at 2011-03-08 21:13:10
41. Very Good
View Tutorial By: Lalit at 2011-02-23 12:24:13
42. well example, but i have to send the received list from servlet to jsp , to servlet again
View Tutorial By: amit kumar at 2011-02-21 00:43:15
43. just want to ask..how can i use iterator to compare value inside the object...for example..finding h
View Tutorial By: ikhwan at 2011-02-02 17:14:07
44. Thanks a lot!! Just looking for such example
View Tutorial By: Manuel at 2011-01-20 08:17:00
45. Is there any difference between Iterator and ListIterator?
View Tutorial By: Ram at 2011-01-02 21:58:48
46. Thanks for this article. Now I can understand Iterator better.
View Tutorial By: Rajesh Kumar at 2010-12-14 03:27:25
47. superb...thanks :-)
View Tutorial By: sunny at 2010-11-10 07:20:25
48. Thanks for giving iterator example
View Tutorial By: Rajesh Kumar at 2010-11-03 05:17:05
49. thanks a lot
View Tutorial By: vipul at 2010-10-18 10:23:56
50. good, it clear the confusion......
nice code, but not good to initalize the object in the whi
View Tutorial By: shree at 2010-10-15 00:57:42
51. PROGRAMMING SUPERB
View Tutorial By: vinay sharma at 2010-10-01 05:16:23
52. Really this code helped with some idea...
View Tutorial By: Priya at 2010-09-29 22:21:46
53. Good information...it help me to do mi projecto! thanks.
View Tutorial By: gnk at 2010-08-12 01:01:50
54. it's a usefule example for who's learning java
View Tutorial By: mathhoang at 2010-07-14 08:29:43
55. good one
View Tutorial By: kumar saurabh at 2010-06-20 13:29:50
56. its realy gud which clears minor douts regarding the problems we are facing
View Tutorial By: Shripad at 2010-01-19 05:40:41
57. thks for help...
View Tutorial By: Peps at 2010-01-05 02:34:15
58. thks for help...
View Tutorial By: Peps at 2010-01-05 02:28:52
59. i know this example this is in mg.hill but i want to know that with out set by ListIterator cant we
View Tutorial By: srikanta at 2009-11-18 00:06:57
60. sir ji,
i want do learn for loop in java and form which i want to create different shapes lik
View Tutorial By: Prabhakar at 2009-11-06 00:48:53
61. The iterator returned by iterator() does not point to start of of collection. It points to head whic
View Tutorial By: Jeffrey at 2009-10-06 21:45:41
62. could u pls tell me whcih class is implemted by iterator interface...
if any class implements
View Tutorial By: AMIT at 2009-10-04 00:21:30
63. Nice!
View Tutorial By: xeon at 2009-09-30 01:24:05
64. Sorry:
#!/usr/bin/env ruby
al = ["C", "A", "E",
View Tutorial By: puma at 2009-09-23 16:39:57
65. tnx, nice code...
View Tutorial By: Hansoc at 2009-08-18 00:17:58
66. I'm a beginner, and I was told it is a good practice to use:
List <String>list
View Tutorial By: Nelson at 2009-08-17 09:13:28
67. Thanks, much appreciated!
Saved a great deal of trawling thru- the classes and interfaces in
View Tutorial By: alan at 2009-08-14 01:35:01
68. thats good
View Tutorial By: P.Kumar at 2009-08-02 10:48:48
69. just what i needed to understand iterators, gj
View Tutorial By: oak at 2009-04-20 15:46:39
70. tnx .
View Tutorial By: smokes at 2009-04-15 07:30:19
71. u doing good
View Tutorial By: matsiko at 2009-04-14 08:05:00
72. Thank for your clear explanation
View Tutorial By: htnga at 2009-04-05 18:43:25
73. good understandability code
View Tutorial By: Raja Mahendra Kumar at 2009-03-05 03:11:26
74. Great Work for JAVA Beginners
View Tutorial By: Sudhir Rajgure at 2009-03-01 02:40:00
75. Thankyou it is Working Fine .....
View Tutorial By: PDCOE at 2009-02-11 23:02:29
76. Good explanation about Iterator. But Object declaration inside while loop is not all good style of p
View Tutorial By: Prasad at 2008-12-22 23:20:59
77. Timely info right when I needed it. Thanks.
View Tutorial By: Electric Vehicles at 2008-12-10 15:15:25
78. good quick explanation. Thanks
View Tutorial By: kay at 2008-12-08 21:10:13
79. good
View Tutorial By: sasikiran at 2008-03-30 23:33:24
80. very good for programing
View Tutorial By: kiran at 2007-12-06 00:39:47
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