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    <p>First - I have to direct you to <a href="http://www.angelikalanger.com/GenericsFAQ/JavaGenericsFAQ.html" rel="noreferrer">http://www.angelikalanger.com/GenericsFAQ/JavaGenericsFAQ.html</a> -- she does an amazing job.</p> <p>The basic idea is that you use</p> <pre><code>&lt;T extends SomeClass&gt; </code></pre> <p>when the actual parameter can be <code>SomeClass</code> or any subtype of it.</p> <p>In your example,</p> <pre><code>Map&lt;String, Class&lt;? extends Serializable&gt;&gt; expected = null; Map&lt;String, Class&lt;java.util.Date&gt;&gt; result = null; assertThat(result, is(expected)); </code></pre> <p>You're saying that <code>expected</code> can contain Class objects that represent any class that implements <code>Serializable</code>. Your result map says it can only hold <code>Date</code> class objects.</p> <p>When you pass in result, you're setting <code>T</code> to exactly <code>Map</code> of <code>String</code> to <code>Date</code> class objects, which doesn't match <code>Map</code> of <code>String</code> to anything that's <code>Serializable</code>.</p> <p>One thing to check -- are you sure you want <code>Class&lt;Date&gt;</code> and not <code>Date</code>? A map of <code>String</code> to <code>Class&lt;Date&gt;</code> doesn't sound terribly useful in general (all it can hold is <code>Date.class</code> as values rather than instances of <code>Date</code>)</p> <p>As for genericizing <code>assertThat</code>, the idea is that the method can ensure that a <code>Matcher</code> that fits the result type is passed in.</p>
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    1. POWhen do Java generics require <? extends T> instead of <T> and is there any downside of switching?
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    1. USScott Stanchfield
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    1. POWhen do Java generics require <? extends T> instead of <T> and is there any downside of switching?
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    1. COIn this case, yes I do want a map of classes. The example I gave is contrived to use standard JDK classes rather than my custom classes, but in this case the class is actually instantiated via reflection and used based on the key. (A distributed app where the client doesn't have the server classes available, just the key of which class to use to do the server side work).
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    2. COI guess where my brain is stuck is on why a Map containing classes of type Date doesn't just fit nicely into a type of Maps containing classes of type Serializable. Sure the classes of type Serializable could be other classes as well, but it certainly includes type Date.
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    3. COOn the assertThat making sure the cast is performed for you, the matcher.matches() method doesn't care, so since the T is never used, why involve it? (the method return type is void)
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