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PORecursive generic class definition and "cannot be converted" compilation error
 

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  1. PORecursive generic class definition and "cannot be converted" compilation error
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    <p>This is a design similar to other JPA BaseEntity patterns you may have seen:</p> <pre><code>@MappedSuperclass() public abstract class Entity&lt;X extends Entity&lt;X&gt;&gt; implements Comparable&lt;X&gt;, Serializable { private static final long serialVersionUID = 1L; private Long id; private Date timeStamp; ... // Simply compare fields in subclass until difference is discovered private int compareSubclassFields(X that) { int result = 0; for(Comparator&lt;X&gt; comparator : getComparators()) { result = comparator.compare(this,that); &lt;&lt;=== compilation error if(result != 0) { break; } } return result; } /** * Entity subclasses provide a list of their own special * comparators that can contribute to the comparison. */ protected abstract List&lt;Comparator&lt;X&gt;&gt; getComparators(); } </code></pre> <p>Here is an example of a class that extends Entity:</p> <pre><code>public class User extends Entity&lt;User&gt; { private static final long serialVersionUID = 1L; private String firstName; private String lastName; private String email; ... @Override public List&lt;Comparator&lt;User&gt;&gt; getComparators() { List&lt;Comparator&lt;User&gt;&gt; result = new ArrayList&lt;Comparator&lt;User&gt;&gt;(); result.add(getLastNameComparator()); // Sort first on last name result.add(getFirstNameComparator());// Next, by first name result.add(getEmailComparator()); // Finally, by email (unique) return result; } } </code></pre> <p>When I compile, I get the following error:</p> <pre><code>error: method compare in interface Comparator&lt;T&gt; cannot be applied to given types; result = comparator.compare(this,that); ^ required: X,X found: Entity&lt;X&gt;,X reason: actual argument Entity&lt;X&gt; cannot be converted to X by method invocation conversion where X,T are type-variables: X extends Entity&lt;X&gt; declared in class Entity T extends Object declared in interface Comparator </code></pre> <p>Reading <a href="https://stackoverflow.com/questions/211143/java-enum-definition">Java Enum Definition</a>, in particular the part where it says,</p> <pre><code>public class StatusCode extends Enum&lt;StatusCode&gt; </code></pre> <blockquote> <p>Now if you check the constraints, we've got Enum - so E=StatusCode. Let's check: does E extend Enum? Yes! We're okay. </p> </blockquote> <p>I assume that in my example, where <code>X extends Entity&lt;X&gt;</code>, 'this' would be an instance of <code>User</code> and not <code>Entity&lt;User&gt;</code>. Moreover, because Entity is an abstract class it <em>must</em> be extended and, therefore, <code>compareNonIdFields</code> can only be invoked by an instance of X -- on itself. Of course, when I cast I get the unchecked warning:</p> <pre><code>warning: [unchecked] unchecked cast result = comparator.compare(((X)this),that); ^ required: X found: Entity&lt;X&gt; where X is a type-variable: X extends Entity&lt;X&gt; declared in class Entity 1 warning </code></pre> <p>Thoughts on why this recursive generic usage causes a compilation error and solutions to make the unchecked cast warning go away would be greatly appreciated.</p>
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