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POGuava: ImmutableList magic of the copyOf() method
 

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  1. POGuava: ImmutableList magic of the copyOf() method
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    <p>I would like to feel the 'magic power' of the <code>copyOf()</code> method of Guava <code>guava-libraries</code>. </p> <p>There is small app that I use to check it. </p> <p>Here is the <a href="https://code.google.com/p/guava-libraries/wiki/ImmutableCollectionsExplained" rel="nofollow noreferrer">documentation</a>:</p> <blockquote> <p>The JDK provides <code>Collections.unmodifiableXXX</code> methods, but in our opinion, these can be</p> <ul> <li>unwieldy and verbose; unpleasant to use everywhere you want to make defensive copies</li> <li>unsafe: the returned collections are only truly immutable if nobody holds a reference to the original collection</li> </ul> </blockquote> <p>So, I try to build a model where <code>"someone holds a reference to the original collection"</code>. Thus, working with a copy of collection I should not be worried about changing value on copy. But magic does not work so far (there are two tries: 1. <code>copyOf(collection)</code>, 2. <code>copyOf(iterator)</code>):</p> <pre><code>import com.google.common.collect.ImmutableList; import java.util.LinkedList; import java.util.List; class MyObject { String name; public MyObject(String name) {this.name = name;} @Override public String toString() { return name; } } public class ListUnsafe { List&lt;MyObject&gt; list = new LinkedList&lt;MyObject&gt;(); { list.add(new MyObject("a")); list.add(new MyObject("b")); list.add(new MyObject("c")); } public List&lt;MyObject&gt; getList() { return ImmutableList.copyOf(list); } public List&lt;MyObject&gt; getCopyIterator() { return ImmutableList.copyOf(list.iterator()); } public static void main(String[] args) { ListUnsafe obj = new ListUnsafe(); { MyObject ref = obj.list.get(0); List&lt;MyObject&gt; myList = obj.getList(); MyObject copyObj = myList.get(0); copyObj.name = "new"; System.out.println("ref: " + ref); } obj = new ListUnsafe(); { MyObject ref = obj.list.get(0); List&lt;MyObject&gt; myList = obj.getCopyIterator(); MyObject copyObj = myList.iterator().next(); copyObj.name = "new"; System.out.println("ref: " + ref); } } } </code></pre> <p>The output:</p> <pre><code>ref: new ref: new </code></pre> <p>It means that we changed original data. What we did not want.</p> <h2>Question</h2> <p>Why it does not do copy?<br> How it differs from <code>unmodifiableXXX</code>?</p> <p>There is <a href="https://stackoverflow.com/questions/9517228/guava-immutablelist-copyof-vs-builder">link</a> to similar question:</p> <p>The answer says about <code>copyOf</code>:</p> <ul> <li>(from source) <code>copyOf(Collection)</code> instance doesn't create temporary <code>ArrayList</code> (<code>copyOf(Iterable)</code> and <code>copyOf(Iterator)</code> do so).</li> </ul>
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    1. COPresumably the list itself is immutable; the contained elements aren't.
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      1. USOliver Charlesworth
    2. CONote: the Collections.unmodifiable*() methods are not intended to return an immutable instance - they are intended to return a view of the collection that cannot be modified through that view. This can be valuable when the provider of the data needs to control modifications.
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      1. USAndy Thomas
    3. COThey compare guava to jdk's unmodifiableXXX. Is if it could resolve issue if someone "holds a reference to the original collection" and this ref would hold old unchangeable value. As I understand it. How it differs from unmodifiableXXX?
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