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    <p>Iteration over collections in Java is not thread safe, even if you are using one of the synchronized wrappers (<code>Collections.synchronizedMap(...)</code>):</p> <blockquote> <p>It is imperative that the user manually synchronize on the returned map when iterating over any of its collection views:</p> <pre><code>Map m = Collections.synchronizedMap(new HashMap()); ... Set s = m.keySet(); // Needn't be in synchronized block ... synchronized(m) { // Synchronizing on m, not s! Iterator i = s.iterator(); // Must be in synchronized block while (i.hasNext()) foo(i.next()); } </code></pre> </blockquote> <p><a href="http://docs.oracle.com/javase/8/docs/api/java/util/Collections.html#synchronizedMap-java.util.Map-" rel="nofollow noreferrer">Java Collection Framework docs</a></p> <p>Other calls to synchronized collections are safe, as the wrapper classes surround them with <code>synchronized</code> blocks, which use the wrapper collection as their monitor:</p> <pre><code>public int size() { synchronized( this ) { return collection.size(); } } </code></pre> <p>with <code>collection</code> being the original collection. This works for all methods exposed by a collection/map, except for the iteration stuff.</p> <p>The key set of a map is made synchronized just the same way: the synchronized wrapper does not return the original key set at all. Instead, it returns a special synchronized wrapper of the collection's original key set. The same applies to the entry set and the value set.</p>
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    1. POExplain synchronization of collections when iterators are used?
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    1. COFixed the answer: the monitor used is actually the wrapper collection, not the original one.
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    2. COThat is super helpful and presented very well. I was having trouble finding a source that explains this clearly, so thanks very much!
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    3. CO"Iteration over collections in Java is not thread safe, even if you are using one of the synchronized wrappers" O_o terrible
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