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POWhen a value is added via Map.put(K, V), must the same instance be returned via Map.get(K)?
 

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  1. POWhen a value is added via Map.put(K, V), must the same instance be returned via Map.get(K)?
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    <p>Suppose you have this code:</p> <pre><code>Map&lt;Foo, Bar&gt; map = new HashMap&lt;Foo, Bar&gt;(); Foo foo = new Foo(); Bar bar = new Bar(); map.put(foo, bar); Bar barReturned = map.get(foo); </code></pre> <p>Does Java require that <code>barReturned == bar</code>? That is, does Java <em>require</em> that <code>barReturned</code> be <em>the same instance</em> as <code>bar</code>? If not, what semantics are expected?</p> <p>The <a href="http://download.oracle.com/javase/6/docs/api/java/util/Map.html" rel="nofollow">Javadoc</a> suggests that <code>barReturned == bar</code> must be true, but I'm not 100% sure:</p> <blockquote> <p><strong><code>V get(Object key)</code></strong></p> <p>Returns the value to which the specified key is mapped, or <code>null</code> if this map contains no mapping for the key.</p> <p>More formally, if this map contains a mapping from a key <code>k</code> to a value <code>v</code> such that <code>(key==null ? k==null : key.equals(k))</code>, then this method returns <code>v</code>; otherwise it returns <code>null</code>. (There can be at most one such mapping.)</p> <p>If this map permits <code>null</code> values, then a return value of null does not <em>necessarily</em> indicate that the map contains no mapping for the key; it's also possible that the map explicitly maps the key to <code>null</code>. The containsKey operation may be used to distinguish these two cases.</p> <p><strong>Parameters:</strong></p> <p><code>key</code> - the key whose associated value is to be returned</p> <p><strong>Returns:</strong></p> <p><em><strong>THE VALUE TO WHICH THE SPECIFIED KEY IS MAPPED</em></strong>, or <code>null</code> if this map contains no mapping for the key</p> </blockquote> <p>(Emphasis mine)</p> <p><strong>Edit:</strong> I understand that the implementations of <code>Map</code> that come bundled with the standard library adhere to the <code>barReturned == bar</code> semantics. What I want to know is whether or not this behavior is <strong>required</strong> as per the documentation. For example, must I also adhere to these semantics if I write my own class that implements <code>Map</code>?</p>
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