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POWhen do Java generics require <? extends T> instead of <T> and is there any downside of switching?
 

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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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    <p>Given the following example (using JUnit with Hamcrest matchers):</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>This does not compile with the JUnit <code>assertThat</code> method signature of:</p> <pre><code>public static &lt;T&gt; void assertThat(T actual, Matcher&lt;T&gt; matcher) </code></pre> <p>The compiler error message is:</p> <pre><code>Error:Error:line (102)cannot find symbol method assertThat(java.util.Map&lt;java.lang.String,java.lang.Class&lt;java.util.Date&gt;&gt;, org.hamcrest.Matcher&lt;java.util.Map&lt;java.lang.String,java.lang.Class &lt;? extends java.io.Serializable&gt;&gt;&gt;) </code></pre> <p>However, if I change the <code>assertThat</code> method signature to:</p> <pre><code>public static &lt;T&gt; void assertThat(T result, Matcher&lt;? extends T&gt; matcher) </code></pre> <p>Then the compilation works.</p> <p>So three questions:</p> <ol> <li>Why exactly doesn't the current version compile? Although I vaguely understand the covariance issues here, I certainly couldn't explain it if I had to.</li> <li>Is there any downside in changing the <code>assertThat</code> method to <code>Matcher&lt;? extends T&gt;</code>? Are there other cases that would break if you did that?</li> <li>Is there any point to the genericizing of the <code>assertThat</code> method in JUnit? The <code>Matcher</code> class doesn't seem to require it, since JUnit calls the matches method, which is not typed with any generic, and just looks like an attempt to force a type safety which doesn't do anything, as the <code>Matcher</code> will just not in fact match, and the test will fail regardless. No unsafe operations involved (or so it seems).</li> </ol> <p>For reference, here is the JUnit implementation of <code>assertThat</code>:</p> <pre><code>public static &lt;T&gt; void assertThat(T actual, Matcher&lt;T&gt; matcher) { assertThat("", actual, matcher); } public static &lt;T&gt; void assertThat(String reason, T actual, Matcher&lt;T&gt; matcher) { if (!matcher.matches(actual)) { Description description = new StringDescription(); description.appendText(reason); description.appendText("\nExpected: "); matcher.describeTo(description); description .appendText("\n got: ") .appendValue(actual) .appendText("\n"); throw new java.lang.AssertionError(description.toString()); } } </code></pre>
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