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POCovariant Typeparameter in scala needs to be invariant in java interface
 

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    <p>I've got a trait that looks like this (some further information can be found at <a href="https://stackoverflow.com/questions/3695990/inheritance-and-automatic-type-conversion">this related question by myself</a> although I don't think, it's needed for this question)</p> <pre><code>trait Extractor[-A,+B] { def extract(d:A):B //lots of other things } </code></pre> <p>To use this in an existing java framework I would like this <code>Extractor</code> to either have a function that returns a <code>Comparator[B]</code> (being <a href="http://download.oracle.com/javase/6/docs/api/java/util/Comparator.html" rel="nofollow noreferrer">java.util.Comparator</a>) or even better extend <code>Comparator[A]</code>. Now that poses a problem because <code>Comparator</code>s type parameter is ought to be invariant, while <code>A</code> is contravariant and <code>B</code> is covariant.</p> <p>So I get errors like this:</p> <pre><code>scala&gt; import java.util.Comparator import java.util.Comparator scala&gt; trait Extractor[-A,+B] extends Comparator[A] &lt;console&gt;:6: error: contravariant type A occurs in invariant position in type [-A,+B]java.lang.Object with java.util.Comparator[A] of trait Extractor trait Extractor[-A,+B] extends Comparator[A] ^ scala&gt; trait Extractor[-A, +B] { | def comp:Comparator[B] | } &lt;console&gt;:7: error: covariant type B occurs in invariant position in type =&gt; java.util.Comparator[B] of method comp def comp:Comparator[B] ^ </code></pre> <p>Do you see any way out of this or is this just one of those cases where "using java generics in scala hurts"?</p>
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