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POJava multiple generic collection parameters compile error
 

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    <p>So strange! Please have a look the code first:</p> <pre><code>public class A {} public class B extends A {} public class C extends A {} public class TestMain { public &lt;T extends A&gt; void test(T a, T b) {} public &lt;T extends A&gt; void test(List&lt;T&gt; a, List&lt;T&gt; b) {} public void test1(List&lt;? extends A&gt; a, List&lt;? extends A&gt; b) {} public static void main(String[] args) { new TestMain().test(new B(), new C()); new TestMain().test(new ArrayList&lt;C&gt;(), new ArrayList&lt;C&gt;()); new TestMain().test(new ArrayList&lt;B&gt;(), new ArrayList&lt;C&gt;()); new TestMain().test1(new ArrayList&lt;B&gt;(), new ArrayList&lt;C&gt;()); } } </code></pre> <p>The statement <code>new TestMain().test(new ArrayList&lt;B&gt;(), new ArrayList&lt;C&gt;())</code> gets a compilation error:</p> <blockquote> <p>Bound mismatch: The generic method test(T, T) of type TestMain is not applicable for the arguments <code>(ArrayList&lt;B&gt;, ArrayList&lt;C&gt;)</code>. The inferred type <code>ArrayList&lt;? extends A&gt;</code> is not a valid substitute for the bounded parameter <code>&lt;T extends A&gt;</code></p> </blockquote> <p>However:</p> <pre><code> new TestMain().test(new B(), new C()) --&gt; compiled ok new TestMain().test(new ArrayList&lt;C&gt;(), new ArrayList&lt;C&gt;()) --&gt; compiled ok new TestMain().test1(new ArrayList&lt;B&gt;(), new ArrayList&lt;C&gt;()) --&gt; compiled ok </code></pre> <p>If we define the generic before the method name, it seems the type of the second generic <code>List</code> parameter must be the same as that of the first. But there is no restriction if we define generic in parameters.</p> <p>Is it a feature or a bug of the compile program? Is there some documentation about it?</p>
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