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POHow do I fix this Java generics wildcard error?
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  1. POHow do I fix this Java generics wildcard error?
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    copied!<p>In this <a href="https://stackoverflow.com/questions/620934/wildcards-and-generics-error">question</a>, TofuBeer was having problems creating a genericized <code>IterableEnumeration</code>.</p> <p>The answer came from jcrossley3 pointing to this link <a href="http://www.javaspecialists.eu/archive/Issue107.html" rel="nofollow noreferrer">http://www.javaspecialists.eu/archive/Issue107.html</a> which pretty much solved the problem. </p> <p>There is still one thing I don't get. The real problem, as effectively pointed out by erickson, was that:</p> <blockquote> <p><em>You cannot specify a wildcard when constructing a parameterized type</em></p> </blockquote> <p>But removing the wildcard in the declaration didn't work either:</p> <pre><code>final IterableEnumeration&lt;ZipEntry&gt; iteratable = new IterableEnumeration&lt;ZipEntry&gt;(zipFile.entries()); </code></pre> <p>Results in the following error: </p> <pre><code>Main.java:19: cannot find symbol symbol : constructor IterableEnumeration(java.util.Enumeration&lt;capture#469 of ? extends java.util.zip.ZipEntry&gt;) location: class IterableEnumeration&lt;java.util.zip.ZipEntry&gt; final IterableEnumeration&lt;ZipEntry&gt; iteratable = new IterableEnumeration&lt;ZipEntry&gt;( zipFile.entries()); ^ 1 error </code></pre> <p>But the samples in the JavaSpecialist do work:</p> <pre><code> IterableEnumeration&lt;String&gt; ie = new IterableEnumeration&lt;String&gt;(sv.elements()); </code></pre> <p>The only difference I can spot is that in the JavaSpecialists blog, the <code>Enumeration</code> comes from a <code>Vector</code> whose signature is: </p> <pre><code>public Enumeration&lt;E&gt; elements() </code></pre> <p>while the one that fails comes from <code>ZipFile</code> whose signature is:</p> <pre><code>public Enumeration&lt;? extends ZipEntry&gt; entries() </code></pre> <p>Finally, all of this is absorbed by the for-each construct and the static make method suggested in the link</p> <pre><code>for(final ZipEntry entry : IterableEnumeration.make( zipFile.entries() )) { if(!(entry.isDirectory())) { names.add(entry.getName()); } } </code></pre> <p>But!! the point in that newsletter was not to solve this problem, but to avoid the need to specify a generic type, just because the syntax looks ugly!!</p> <p>So.. my questions is:</p> <h1>What is happening?</h1> <p>Why doesn't creating an instance of <code>IterableEnumeration</code> work when the parameter is an <code>Enumeration</code> whose type is <code>&lt;? extends SomeClass&gt;</code> ? And why does the make for-each construct swallow the problem?!!!</p> <p>Why does this work:</p> <pre><code>for(final ZipEntry entry : IterableEnumeration.make( zipFile.entries() )) { </code></pre> <p>but this not work?</p> <pre><code>final IterableEnumeration&lt;ZipEntry&gt; iteratable = IterableEnumeration.make( zipFile.entries() ); </code></pre> <p>Below is a (slightly) modified version of TofuBeer's original code:</p> <pre><code>import java.util.Enumeration; import java.util.HashSet; import java.util.Iterator; import java.util.Set; import java.util.zip.ZipEntry; import java.util.zip.ZipFile; import java.util.Vector; public class Main { private ZipFile zipFile; public Set&lt;String&gt; entries() { final Vector&lt;ZipEntry&gt; vector = new Vector&lt;ZipEntry&gt;(); // why this works. //final IterableEnumeration&lt;ZipEntry&gt; iteratable = new IterableEnumeration&lt;ZipEntry&gt;( vector.elements() ); // but this do not. //final IterableEnumeration&lt;ZipEntry&gt; iteratable = new IterableEnumeration&lt;ZipEntry&gt;( zipFile.entries() ); // nor this final IterableEnumeration&lt;ZipEntry&gt; iteratable = IterableEnumeration.make( zipFile.entries() ); // And what's with the for-each that doesn't care about the type? final Set&lt;String&gt; names = new HashSet&lt;String&gt;(); for(final ZipEntry entry : IterableEnumeration.make( zipFile.entries() )) { if(!(entry.isDirectory())) { names.add(entry.getName()); } } return (names); } } class IterableEnumeration&lt;T&gt; implements Iterable&lt;T&gt; { private final Enumeration&lt;T&gt; enumeration; public IterableEnumeration(final Enumeration&lt;T&gt; e) { enumeration = e; } public Iterator&lt;T&gt; iterator() { return new Iterator&lt;T&gt;() { public boolean hasNext() { return (enumeration.hasMoreElements()); } public T next() { return (enumeration.nextElement()); } public void remove() { throw new UnsupportedOperationException("Cannot remove via an Enumeration"); } }; } // As suggested by http://www.javaspecialists.eu/archive/Issue107.html // but doesn't help with: final IterableEnumeration&lt;ZipEntry&gt; iteratable = IterableEnumeration.make( zipFile.entries() ); public static &lt;T&gt; Iterable&lt;T&gt; make(Enumeration&lt;T&gt; en) { return new IterableEnumeration&lt;T&gt;(en); } } </code></pre> <p>I want to understand it!!</p>
 

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