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POYet another ConcurrentModificationException question
 

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  1. POYet another ConcurrentModificationException question
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    <p>I am currently trying to learn how to properly handle multi-threaded access to Collections, so I wrote the following Java application.</p> <p>As you can see, I create a synchronized ArrayList which I try to access once from within a Thread and once without.</p> <p>I iterate over the ArrayList using a for loop. In order to prevent multiple access on the List at the same time, I wrapped the loop into a synchronized block.</p> <pre><code>public class ThreadTest { Collection&lt;Integer&gt; data = Collections.synchronizedList(new ArrayList&lt;Integer&gt;()); final int MAX = 999; /** * Default constructor */ public ThreadTest() { initData(); startThread(); startCollectionWork(); } private int getRandom() { Random randomGenerator = new Random(); return randomGenerator.nextInt(100); } private void initData() { for (int i = 0; i &lt; MAX; i++) { data.add(getRandom()); } } private void startCollectionWork() { System.out.println("\nStarting to work on data outside of thread"); synchronized (data) { System.out.println("\nEntered synchronized block outside of thread"); for (int value : data) { // ConcurrentModificationException here! if (value % 5 == 1) { System.out.println(value); data.remove(value); data.add(value + 1); } else { System.out.println("value % 5 = " + value % 5); } } } System.out.println("Done working on data outside of thread"); } private void startThread() { Thread thread = new Thread() { @Override public void run() { System.out.println("\nStarting to work on data in a new thread"); synchronized (data) { System.out.println("\nEntered synchronized block in thread"); for (int value : data) { // ConcurrentModificationException if (value % 5 == 1) { System.out.println(value); data.remove(value); data.add(value + 1); } else { System.out.println("value % 5 = " + value % 5); } } } System.out.println("Done working on data in a new thread"); } }; thread.start(); } } </code></pre> <p>But everytime one of the for loops gets entered, I get a ConcurrentModificationException. This is my console output (which changes with every new run):</p> <pre><code>Starting to work on data outside of thread Entered synchronized block outside of thread 51 Starting to work on data in a new thread Entered synchronized block in thread value % 5 = 2 value % 5 = 2 value % 5 = 4 value % 5 = 3 value % 5 = 2 value % 5 = 2 value % 5 = 0 21 Exception in thread "main" java.util.ConcurrentModificationException at java.util.AbstractList$Itr.checkForComodification(AbstractList.java:372) at java.util.AbstractList$Itr.next(AbstractList.java:343) at ThreadTest.startCollectionWork(ThreadTest.java:50) at ThreadTest.&lt;init&gt;(ThreadTest.java:32) at MultiThreadingTest.main(MultiThreadingTest.java:18) Exception in thread "Thread-1" java.util.ConcurrentModificationException at java.util.AbstractList$Itr.checkForComodification(AbstractList.java:372) at java.util.AbstractList$Itr.next(AbstractList.java:343) at ThreadTest$1.run(ThreadTest.java:70) </code></pre> <p>What's wrong?</p> <p>Ps: Please don't just post links to multi-threaded how-to's since I've already read enough about that. I am just curious why my application doesn't run as I want it to.</p> <hr> <p><strong>Update:</strong> I replaced the for(x : y) syntax with an explicit Iterator and a while loop. The problem remains though..</p> <pre><code>synchronized(data){ Iterator&lt;Integer&gt; i = data.iterator(); while (i.hasNext()) { int value = i.next(); // ConcurrentModificationException here! if (value % 5 == 1) { System.out.println(value); i.remove(); data.add(value + 1); } else { System.out.println("value % 5 = " + value % 5); } } } </code></pre>
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