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POJava, Static Method Binding and Generics all rolled up with some Method Overloading
 

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  1. POJava, Static Method Binding and Generics all rolled up with some Method Overloading
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    <p>So as the title implies my question is a bit odd and complicated. I know what I'm about to do breaks all the rules of "good" programming practices but hey, what's life if we don't live a little?</p> <p>So what I did was create the following program. (Not this was part of a larger experiment to really try and understand generics so some of the function names maybe a bit out of order)</p> <pre><code>import java.util.*; public class GenericTestsClean { public static void test2() { BigCage&lt;Animal&gt; animalCage=new BigCage&lt;Animal&gt;(); BigCage&lt;Dog&gt; dogCage=new BigCage&lt;Dog&gt;(); dogCage.add(new Dog()); animalCage.add(new Cat()); animalCage.add(new Dog()); animalCage.printList(dogCage); animalCage.printList(animalCage); } public static void main(String [] args) { //What will this print System.out.println("\nTest 2"); test2(); } } class BigCage&lt;T&gt; extends Cage&lt;T&gt; { public static &lt;U extends Dog&gt; void printList(List&lt;U&gt; list) { System.out.println("*************"+list.getClass().toString()); for(Object obj : list) System.out.println("BigCage: "+obj.getClass().toString()); } } class Cage&lt;T&gt; extends ArrayList&lt;T&gt; { public static void printList(List&lt;?&gt; list) { System.out.println("*************"+list.getClass().toString()); for(Object obj : list) System.out.println("Cage: "+obj.getClass().toString()); } } class Animal { } class Dog extends Animal { } class Cat extends Animal { } </code></pre> <p>Now what is confusing me is that this compiles fine with <strong>javac 1.6.0_26</strong> but when I run it I get the following class cast exception:</p> <pre><code>Test 2 *************class BigCage BigCage: class Dog *************class BigCage Exception in thread "main" java.lang.ClassCastException: Cat cannot be cast to Dog at BigCage.printList(GenericTestsClean.java:31) at GenericTestsClean.test2(GenericTestsClean.java:13) at GenericTestsClean.main(GenericTestsClean.java:21) </code></pre> <p>A number of things to note here:</p> <ol> <li>The two printList are <strong>NOT</strong> overriding but overloading each other as expected(They have different types because the generic types of their arguments are different). This can be verified by using the @Override annotation</li> <li>Changing the <code>void printList(List&lt;?&gt;)</code> method in <strong>class Cage</strong> to be non-static generates an appropriate compile time error</li> <li>Changing the method <code>void &lt;U extends Dog&gt; printList(List&lt;U&gt;)</code> in <strong>class BigCage</strong> to <code>void &lt;U&gt; printList(List&lt;U&gt;)</code> generates an appropriate error.</li> <li>In <strong>main()</strong> calling <strong>printList()</strong> through the <strong>class BigCage</strong> (ie BigCage.printList(...)) generates the same runtime error</li> <li>In <strong>main()</strong> calling <strong>printList()</strong> through the <strong>class Cage</strong> (ie Cage.printList(...)) works as expected only calling the version of <strong>printList</strong> in <strong>Cage</strong></li> <li>If I copy the definition for <code>printList(List&lt;?&gt;)</code> to <strong>class BigCage</strong> from <strong>class Cage</strong>, which will hide the definition in <strong>class Cage</strong>, I get the appropriate compiler error</li> </ol> <p>Now if I had to take a shot in the dark as to what is going on here, I'd say the compiler is screwing up because it's working in multiple phases: <em>Type Checking</em> and <em>Overloaded Method Resolution</em>. During the type checking phase we get through the offending line because <strong>class BigCage</strong> inherited <code>void printList(List&lt;?&gt;)</code> from <code>class Cage</code> which will match any old List we throw at it, so sure we have a method that will work. However once it comes time to resolve with method to actually call we have a problem due to Type Erasure which causes both <code>BigCage.printList</code> and <code>Cage.printList</code> to have the exact same signature. This means when compiler is looking for a match for <code>animalCage.printList(animalCage);</code> it will choose the first method it matches (and if we assume it starts at the bottom with BigCage and works its why up to Object) it'll find <code>void &lt;U extends Dog&gt; printList(List&lt;U&gt;)</code> first instead of the correct match <code>void printList(List&lt;?&gt;)</code></p> <p><strong>Now for my real question</strong>: How close to the truth am I here? Is this a known bug? Is this a bug at all? I know how to get around this problem, this is more of an academic question.</p> <blockquote> <p><strong>**EDIT**</strong></p> <p>As few people have posted below, this code will work in Eclipse. My specific question deals with javac version 1.6.0_26. Also, I'm not sure if I completely agree with Eclipse in this case, even though it works, because adding a <code>printList(List&lt;?&gt;)</code> to <strong>BigCage</strong> will result in a compile time error in Eclipse and I can't see reason why it should work when the same method is inherited verses manually added (See <strong>Note 6</strong> above).</p> </blockquote>
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    1. COIANALL, but note 3, above, seems like a smoking gun - that it looks like a bug. On the other hand, this stuff is always messy - C++ templates have the same kinds of problem - only more.
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    2. COI copied the same code in Eclipse and ran it. It worked perfectly. No Exceptions. I have copied all the classes in the same file(which I know is a bad practice, but I was just checking your question).
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    3. CO@Logan I have edited the original question to better reflect the point you are making about Eclipse vs javac
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