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    <blockquote> <p>only one who can possibly modify this list is the Constructor of the class holding it </p> </blockquote> <p>In this case you don't need to modify it at all. I suggest you make it always immutable and you won't get this issue.</p> <p>In your constructor you can do</p> <pre><code>List&lt;HierarchyNode&gt; children = new ArrayList&lt;&gt;(); // create/modify collection. this.children = Collections.immutableList(children); </code></pre> <p>This simplifies your method</p> <pre><code>public List&lt;HierarchyNode&gt; getChildren() { return children; } </code></pre> <p>If you make these changes, I suspect you will find where the collection is being modified.</p> <hr> <p>The problem is that <code>Collections.unmodifiableList()</code> prevents modification using the reference returned by this method. It doesn't prevent modification to the collection you are wrapping.</p> <pre><code>List&lt;String&gt; words = new ArrayList&lt;String&gt;(); words.add("hello"); words.add("world"); List&lt;String&gt; unmodifiable = Collections.unmodifiableList(words); List&lt;String&gt; copy = new ArrayList&lt;String&gt;(words); System.out.println("Before modification"); System.out.println("words: " + words); System.out.println("unmodifiable: " + unmodifiable); System.out.println("copy: " + copy); words.remove("hello"); words.add("hi"); System.out.println("\nAfter modification"); System.out.println("words: " + words); System.out.println("unmodifiable: " + unmodifiable); System.out.println("copy: " + copy); </code></pre> <p>prints</p> <pre><code>Before modification words: [hello, world] unmodifiable: [hello, world] copy: [hello, world] After modification words: [world, hi] unmodifiable: [world, hi] copy: [hello, world] </code></pre> <p>The solution without adding a new library, is to copy the collection before returning it.</p>
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    1. POConcurrentModificationException in single Threaded unmodifiableList
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    1. USPeter Lawrey
    1. USPeter Lawrey
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    1. COI have checked this: the only one who can possibly modify this list is the Constructor of the class holding it as a member. All others, including subclasses can access the list only by using this unmodifiable list:
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      1. USBennyBarns
    2. COIn that case, see my answer on how to prevent modification and possibly detect where it is being modified.
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