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POWhy doesn't Java Map extends Collection?
 

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    <p>I was surprised by the fact that <code>Map&lt;?,?&gt;</code> is not a <code>Collection&lt;?&gt;</code>.</p> <p>I thought it'd make a LOT of sense if it was declared as such:</p> <pre><code>public interface Map&lt;K,V&gt; extends Collection&lt;Map.Entry&lt;K,V&gt;&gt; </code></pre> <p>After all, a <code>Map&lt;K,V&gt;</code> is a collection of <a href="http://java.sun.com/javase/6/docs/api/java/util/Map.Entry.html" rel="noreferrer"><code>Map.Entry&lt;K,V&gt;</code></a>, isn't it?</p> <p>So is there a good reason why it's not implemented as such?</p> <hr> <p>Thanks to Cletus for a most authoritative answer, but I'm still wondering why, if you can already view a <code>Map&lt;K,V&gt;</code> as <code>Set&lt;Map.Entries&lt;K,V&gt;&gt;</code> (via <code>entrySet()</code>), it doesn't just extend that interface instead.</p> <blockquote> <p>If a <code>Map</code> is a <code>Collection</code>, what are the elements? The only reasonable answer is "Key-value pairs"</p> </blockquote> <p>Exactly, <code>interface Map&lt;K,V&gt; extends Set&lt;Map.Entry&lt;K,V&gt;&gt;</code> would be great!</p> <blockquote> <p>but this provides a very limited (and not particularly useful) <code>Map</code> abstraction.</p> </blockquote> <p>But if that's the case then why is <code>entrySet</code> specified by the interface? It must be useful somehow (and I think it's easy to argue for that position!).</p> <blockquote> <p>You can't ask what value a given key maps to, nor can you delete the entry for a given key without knowing what value it maps to.</p> </blockquote> <p>I'm not saying that that's all there is to it to <code>Map</code>! It can and <em>should</em> keep all the other methods (except <code>entrySet</code>, which is redundant now)!</p>
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    1. COevent Set<Map.Entry<K,V>> actually
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    2. COIt just doesn't. It's based on an opinion (as described in the Design FAQ), rather than being a logical conclusion. For an alternative approach look at the design of containers in the C++ STL (http://www.sgi.com/tech/stl/table_of_contents.html) which is based upon Stepanov's thorough and brilliant analysis.
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