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copied!<p>Currently, the bound on your <code>Action</code> class says - <code>U extends User<Principal></code>. Since <code>MyUser</code> deoesn't extend a <code>User<Principal></code>, but a <code>User<MyPrincipal></code>, it won't work as you expected. A <code>User<MyPrincipal></code> is not a <code>User<Principal></code> (distinct types).</p> <p>You can introduce another type parameter in your <code>Action</code> class, and use that in bounds of <code>U</code>:</p> <pre><code>public abstract class Action<P extends Principal, U extends User<P>> { } </code></pre> <p>and now, you can create your <code>MyAction</code> like this:</p> <pre><code>public class MyAction extends Action<MyPrincipal, MyUser>{ } </code></pre> <hr /> <p>Or you can also modify your class using wildcard bound:</p> <pre><code>public abstract class Action<U extends User<? extends Principal>> { } </code></pre> <p>This way you won't need another type parameter:</p> <pre><code>public class MyAction extends Action<MyUser>{ } </code></pre> <hr /> <p><strong>References:</strong></p> <ul> <li><a href="http://www.angelikalanger.com/GenericsFAQ/JavaGenericsFAQ.html" rel="nofollow">Java Generics FAQs</a> <ul> <li><a href="http://www.angelikalanger.com/GenericsFAQ/FAQSections/TypeParameters.html#Can%20I%20use%20a%20type%20parameter%20as%20part%20of%20its%20own%20bounds%20or%20in%20the%20declaration%20of%20other%20type%20parameters?" rel="nofollow">Can I use a type parameter as part of its own bounds?</a></li> </ul></li> </ul>

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