How to concurrently modify a Vector

Question

I have to ensure while iterating the Vector; there is no update on that Vector to avoid ConcurrentModificationException. I can use concurrent collection. But i just want to give a try on Vector. Below is the code I have written.

public class TestConcurrentModification1 {
    Vector a = new Vector();

    public static void main(String[] args)  {
        final TestConcurrentModification1 obj = new TestConcurrentModification1();

        new Thread(){
            public  void run(){
                for(int i = 0; i < 5; i++){
                    try {
                        Thread.sleep(1);
                    } catch (InterruptedException e) {}
                    obj.a.add(""+i);                
                }
                System.out.println(obj.a);
            }
        }.start();

        new Thread(){
            public  void run(){
                try {
                    Thread.sleep(10);
                    } catch (InterruptedException e) {
                    }
                synchronized (obj.a) {
                    Iterator itr = obj.a.iterator();
                    while(itr.hasNext()) {
                        obj.a.add("TEST");//java.lang.OutOfMemoryError: Java heap space
                        //itr.remove(); //java.lang.IllegalStateException
                    }
                }
            }
        }.start();
    }
}

But the above code is throwing 1) OutOfMemoryError OR 2) IllegalStateException. Could you please explain what is causing these two exception. And how to achieve my goal of avoiding ConcurrentModificationException on a Vector?

I have to solve this for Java 1.4.2 or prior.

Do not use `Vector`. Instead, choose a proper class from `java.util.concurrent` package. — Luiggi Mendoza, Oct 15 '13 at 14:41
`ConcurrentModificationException` is not necessarily for concurrent as in multi-`Thread`ed. You cannot add/remove from a collection while iterating over it. You can remove if you use the `Iterator` itself. — Sotirios Delimanolis, Oct 15 '13 at 14:42
Also, pay a good read on this Q/A: [Why is Java Vector class considered obsolete or deprecated?](http://stackoverflow.com/q/1386275/1065197) — Luiggi Mendoza, Oct 15 '13 at 14:42
@LuiggiMendoza Thanks. But I even mentioned that i want to try with Vector. — Kanagavelu Sugumar, Oct 15 '13 at 14:42
@SotiriosDelimanolis Thanks! I didn't get your comment clearly! Could you please elaborate? — Kanagavelu Sugumar, Oct 15 '13 at 14:45
[Read this](http://stackoverflow.com/questions/7169913/avoiding-concurrentmodificationexception-on-list-by-making-a-shallow-copy) — Sotirios Delimanolis, Oct 15 '13 at 14:46
Team, Thanks for your comments. Could you please also tell me the root cause for those two exceptions/Error in my code ?? — Kanagavelu Sugumar, Oct 15 '13 at 14:48
Every time you add an element, your iterator `next()` will find a new element and loop again. To infinity and beyond! — Sotirios Delimanolis, Oct 15 '13 at 14:50
@SotiriosDelimanolis It is happening for only 10 elements also. I updated my for loop length from 100 to 10. Still error is coming. — Kanagavelu Sugumar, Oct 15 '13 at 15:02
@KanagaveluSugumar Imagine a vector with one element: `if (it.hasNext())` => true `a.add("");` (now the vector has two elements) => `if (it.hasNext())` => true `a.add("")` (now the vector has three elements) etc. That's what your loop is doing... — assylias, Oct 15 '13 at 15:10
@assylias Thanks for your response. But this happens for even with for loop of 2 elements :( — Kanagavelu Sugumar, Oct 15 '13 at 15:14
if i comment out that line "obj.a.add("TEST");" my program is never ending... — Kanagavelu Sugumar, Oct 15 '13 at 15:16
@LuiggiMendoza The above code is very fine which will avoid ConcurrentModificationException while removing.. only i missed .next()... For Adding i should come out from iteration then do add() — Kanagavelu Sugumar, Oct 15 '13 at 15:25

Fildor · Accepted Answer · 2013-10-15T15:23:35.613

2

One part of your Problem:

 Iterator itr = obj.a.iterator();
 while(itr.hasNext()) {
     obj.a.add("TEST");// <- if itr.hasNext() would have returned false in next iteration, now it won't
 }

This is an infinite loop that increases memory usage in each iteration. So you'll run into an OutOfMemory sooner or later.

I suggest using a good old for-loop for inserting values. Use an iterator if you actually want to iterate something :)

More: You are synchronizing against a non-final member.

More: Iterator.remove throws ...

IllegalStateException - if the next method has not yet been called, or the remove method has already been called after the last call to the next method.

And last but not least: The race condition already mentioned by Sotirios (+1 for him). Whenever you synchronize, make sure you synchronize every call on the critical resource.

edited Oct 15 '13 at 15:23

answered Oct 15 '13 at 15:12

Fildor

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No if i comment out that line "obj.a.add("TEST");" my program is never ending... Problem is somewhere else... – Kanagavelu Sugumar Oct 15 '13 at 15:16
Of course it does not, because your iterator never advances! "HasNext" does not change the iterator's position. You have to call `next` inside the loop. – Fildor Oct 15 '13 at 15:18
@KanagaveluSugumar Note that if you call `next()`, you will get a `ConcurrentModificationException`. – Sotirios Delimanolis Oct 15 '13 at 15:21
1

Note that for your last point, `Vector` is already `synchronized` on all (most?) the `public` methods. – Sotirios Delimanolis Oct 15 '13 at 15:29
@SotiriosDelimanolis That, of course, is true. Just wanted to point that out. – Fildor Oct 15 '13 at 15:30

score 2 · Answer 2 · answered Oct 15 '13 at 15:20

You have a good old race condition on your hands.

Your first Thread, except for adding the first element to your Vector, serves absolutely no purpose. You can replace it with

obj.a.add("first");

The beef, as others have noted, is here

Iterator itr = obj.a.iterator();
while (itr.hasNext()) {
    obj.a.add("TEST");// java.lang.OutOfMemoryError: Java
    // heap space
    // itr.remove(); //java.lang.IllegalStateException
}

itr.hasNext() is implemented as

public boolean hasNext() {           
    return cursor != elementCount;
}

Where cursor starts at 0 and elementCount is the size of your Vector. This call will never return false. Your while loop with loop, adding elements, until the program runs out of memory. The cursor never moves forward because you never call next(). If you do call next() while adding elements directly to the Vector, you will get a ConcurrentModificationException.

How to concurrently modify a Vector

2 Answers2

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