Difference between ArrayList and CopyOnWriteArrayList
We know that ArrayList is not thread safe . Hence any simultaneous thread can access and modify the content of list simultaneously. Here lies the problem of ConcurrentModificationException. When one thread is Iterating over an ArrayList and any other thread or the same thread modify the content of list and when we again call next() method of the iterator ,we get ConcurrentModificationException exception.
Lets find the difference between Difference between ArrayList and CopyOnWriteArrayList with the below example .
ArrayList
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import java.util.ArrayList; import java.util.Iterator; public class ArrayListTest { public static void main(String[] args) { ArrayList al=new ArrayList(); al.add("One"); al.add("Two"); al.add("Three"); Iterator it = al.iterator(); while(it.hasNext()){ String str = it.next(); System.out.println(str); al.remove("One"); } } } |
output
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One <span style="color: red;"> Exception in thread "main" java.util.ConcurrentModificationException at java.util.ArrayList$Itr.checkForComodification(Unknown Source) at java.util.ArrayList$Itr.next(Unknown Source) at ArrayListTest.main(ArrayListTest.java:14)</span> |
Here you can notice that we got the ConcurrentModificationException when we called it.next() method. Now lets see why are we getting this exception.
In ArrayList implementation , we have a variable called modCount which holds the modification count and whenever we modify the ArrayList , value of modCount increments. expectedModCount is the iterator variable that is initialized when we create iterator with same value as modCount. Hence iterator throws ConcurrentModificationException if list size is changed.
CopyOnWriteArrayList
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import java.util.Iterator; import java.util.concurrent.CopyOnWriteArrayList; public class ArrayListTest { public static void main(String[] args) { CopyOnWriteArrayList al=new CopyOnWriteArrayList(); al.add("One"); al.add("Two"); al.add("Three"); Iterator it = al.iterator(); while(it.hasNext()){ String str = it.next(); System.out.println(str); al.remove("One"); } } } |
Output
One
Two
Three
Here you can notice that we haven’t got ConcurrentModificationException when we used CopyOnWriteArrayList .
Why CopyOnWriteArrayList is required?
Sometimes we may have to add or remove elements from the list while doing the list iteration, in such case we should use Concurrent Collection class – CopyOnWriteArrayList.
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