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copied!<p>That map is OK; if you really need to add a tiny bit of constraint, just try:</p> <pre><code>Map<Class<? extends EntityProxy>, HashMap<Long, EntityProxy>> indexMap; </code></pre> <p>This would make sure the key class can only be an EntityProxy.class or subclass.</p> <p>You can think of the question mark as some "anything", but anonymous. So <code><?></code> means <em>really anything</em> -- any <code>Object</code>, <code><? extends EntityProxy></code> means anything that fulfils this condition (passes the "<code>instanceof EntityProxy</code>" test).</p> <p>The type safety here is less than desired, as you can still put anything as key and anything in that map. I mean, you can legally put this in the map:</p> <pre><code>indexMap.put(EntityProxy1.class, new HashMap<Long, EntityProxy2>()); </code></pre> <p>(assuming <code>EntityProxy1</code> and <code>EntityProxy2</code> are both subclasses of <code>EntityProxy</code>) since there's no correlation between the key and the value. To enforce that, the <code>put()</code> method of the map would need to be declared like this:</p> <pre><code><T extends EntityProxy> put(Class<T> key, HashMap<Long, T> value); </code></pre> <p><code>T</code> is pretty much like <code>?</code> but the main difference is that it provides you with a name that you can refer to it in that context. So, if I said that <code>?</code> stands for "anything", I would say <code>T</code> stands for "something", as you can refer to that something once you declare it.</p> <p>But you would need a custom data structure for this, as <code>java.util.Map</code> does not provide this kind of constraint. If you're using it as shown in your code sample, I don't think you really need these enforcements.</p>

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