What is ConcurrentModificationException , how is it caused and how can it be prevented ? (Part 2- ArrayLists)

In the previous article, we saw with an example of HashMap  how a ConcurrentModificationException is caused. In this article ,we will see how it happens in case of Lists. The basics are the same as in the example of a HashMap.

 When the structure of a list is modified by adding or removing an element from the list while an iterator is traversing over it, a ConcurrentModificationException is caused. This is because just like HashMap , an ArrayList also maintains a count of the times the structure of the list is modified (modCount). When an iterator is obtained on that list, it creates its own expectedCount which is set to the modCount value . Any addition or removal from the list will change the modCount.When the iterator compares the modCount with expectedCount whenever iterator's next() method is called , it throws the ConcurrentModification exception if it finds a difference.Such an iterator is said to be fail-fast.

 When we use CopyOnWriteArrayList instead of an ArrayList, we don't get this exception. This is because , the iterator of a CopyOnWriteArrayList works on a snapshot of the original list. The iterator may not be able to see the change made to the structure of the original list for this reason.But, it does not fail by throwing the exception and is said to be fail-safe iterator.

 Following is an example of ConcurrentModificationException got on using ArrayList:


 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
package in.blogspot.pallavisonal;

import java.util.ArrayList;
import java.util.Iterator;

public class NotSafeArrayList {
 
 private ArrayList<Integer> myList;
 
 public NotSafeArrayList(ArrayList<Integer> myList){
  this.myList=myList;
 }

 public static void main(String[] args) {
  ArrayList<Integer> aList = new ArrayList<>();
  aList.add(1);
  aList.add(2);
  aList.add(3);
  NotSafeArrayList nsAList = new NotSafeArrayList(aList);
  System.out.println(aList);
  Iterator<Integer> iter = nsAList.myList.iterator();
  while(iter.hasNext()){
   if(iter.next() == 2){
    aList.add(4); 
   }
  }
  System.out.println(aList);
 }

}

Output:
[1, 2, 3]
Exception in thread "main" java.util.ConcurrentModificationException
at java.util.ArrayList$Itr.checkForComodification(ArrayList.java:859)
at java.util.ArrayList$Itr.next(ArrayList.java:831)
at com.explore.pallavi.NotSafeArrayList.main(NotSafeArrayList.java:23)

 The following example shows how we can prevent the ConcurrentModifcation Exception using CopyOnWriteArrayList which uses fail-safe iterators:

 
 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
package in.blogspot.pallavisonal;

import java.util.concurrent.CopyOnWriteArrayList;
import java.util.Iterator;

public class SafeArrayList {
    private CopyOnWriteArrayList<Integer> myList;
 
 public SafeArrayList(CopyOnWriteArrayList<Integer> myList){
  this.myList=myList;
 }

 public static void main(String[] args) {
  CopyOnWriteArrayList<Integer> aList = new CopyOnWriteArrayList<>();
  aList.add(1);
  aList.add(2);
  aList.add(3);
  SafeArrayList nsAList = new SafeArrayList(aList);
  System.out.println(aList);
  Iterator<Integer> iter = nsAList.myList.iterator();
  while(iter.hasNext()){
   if(iter.next() == 2){
    aList.add(4); 
   }
  }
  System.out.println(aList);
 }

}

Output:
[1, 2, 3]
[1, 2, 3, 4]

Comments

Post a Comment

Popular posts from this blog

Day#3 of Java Programming Practice- Longest word of a Sentence

Image
Today's problem is finding longest word of a sentence.The twist being, in case any of the words has punctuation marks, the punctuation should be ignored while calculating the length of that word. My first choice to separate each word in the sentence was by using a String Tokenizer, but when I looked in the documentation for the syntax, it told me that using split is a better idea. This twist of ignoring punctuation mark got me thinking , and this is the program I wrote along with the output. Though it gives correct output, I felt it to be an ugly version. I started looking on the net for a better solution, Though I didn't get an exact solution of this problem, I did get an interesting idea that helped in making the program look better. So here is the second version of my program.  This made me realize that I need to work on learning regular expressions properly. And the learning notes will certainly make it to a post on this blog.  Signing off for tod...
Read more

Book Review: Those Pricey Thakur Girls

Image
Title: Those Pricey Thakur Girls Author: Anuja Chauhan Genre: Fiction Source : Self-purchased Copy Synopsis: The story revolves around the residents of 16, Hailey Road. Retired Judge Lakshmi Narayan Thakur -the man of the house, his wife Mamta Thakur and their five beautiful daughters named alphabetically from A to E. The protagonist is D- Debjani Thakur, who lands up a job as a newsreader in the national channel DD .Dylan Singh Shekhawat- her father's kot-piece partner cum friend's son - unaware of her whereabouts, bashes her thoroughly through his article in the newspaper India Post he is working for. As fate has it, they happen to meet - the twists and turns of events following their meeting is something to be found out on reading the book. Discovery: I had read the first novel from Anuja Chauhan "The Zoya Factor" and had loved it thoroughly. So, recently when I came across this title in one of the book selling sites, I picked it up just because th...
Read more

Day# 5 of Java Programming Practice- Change the first letter of each word in a sentence to Uppercase.

Image
This problem also seemed simple and I didn't take much time to write this one. In fact I decided to move on to the next problem again today and post two of them together. But on reading the next problem I felt that I will be able to do it better once I am through with regular expressions. So, taking a break to read through the topic of regular expressions before diving into the next problem.   Here is the problem and my solution of the day. The problem is just to change the first letter of each word in a sentence to capital case.  
Read more