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    <p>Normally no because you are not changing the internal state of the collection in this case. When you iterate over the collection a new instance of the iterator is created and the state of the iteration is per iterator instance.</p> <hr> <p><strong>Aside note:</strong> Remember that by keeping a read-only collection you are only preventing modifications to the collection itself. Each collection element is still changeable.</p> <pre><code>class Test { public Test(final int a, final int b) { this.a = a; this.b = b; } public int a; public int b; } public class Main { public static void main(String[] args) throws Exception { List&lt;Test&gt; values = new ArrayList&lt;Test&gt;(2); values.add(new Test(1, 2)); values.add(new Test(3, 4)); List&lt;Test&gt; readOnly = Collections.unmodifiableList(values); for (Test t : readOnly) { t.a = 5; } for (Test t : values) { System.out.println(t.a); } } } </code></pre> <p>This outputs:</p> <pre><code>5 5 </code></pre> <hr> <p>Important considerations from @WMR answser.</p> <blockquote> <p>It depends on if the threads that are reading your collection are started before or after you're filling it. If they're started before you fill it, you have no guarantees (without synchronizing), that these threads will ever see the updated values. </p> <p>The reason for this is the Java Memory Model, if you wanna know more read the section "Visibility" at this link: <a href="http://gee.cs.oswego.edu/dl/cpj/jmm.html" rel="nofollow noreferrer">http://gee.cs.oswego.edu/dl/cpj/jmm.html</a></p> <p>And even if the threads are started after you fill your collection, you might have to synchronize because your collection implementation could change its internal state even on read operations (thanks <a href="https://stackoverflow.com/questions/152342/in-java-do-i-need-to-declare-my-collection-synchronized-if-its-read-only#152425">Michael Bar-Sinai</a>, I didn't know such collections existed).</p> <p>Another very interesting read on the topic of concurrency which covers topics like publishing of objects, visibility, etc. in much more detail is Brian Goetz's book <a href="http://www.briangoetz.com/pubs.html" rel="nofollow noreferrer">Java Concurrency in Practice</a>.</p> </blockquote>
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    1. POIn Java, do I need to declare my collection synchronized if it's read-only?
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    1. USJorge Ferreira
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    1. POIn Java, do I need to declare my collection synchronized if it's read-only?
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    1. COThis answer is not wrong but is also not sufficient. The answer from @WMR has some additional important caveats. In particular, you need to consider the issue of making sure that the reader threads will actually see the values written at startup either through safe publication or some memory barrier
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    2. COSee @WMR's and my answers below... this answer is wrong, as get() actions could transform internal state in the collections (e.g. caches, last-touched statistics, etc).
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    3. CO@Alex Miller: I think the answer is not only not sufficient, it clearly says "No" without listing any conditions for this no. And this is wrong (and as you probably know dangerous if you rely on it).
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