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    <p>Currently, the bound on your <code>Action</code> class says - <code>U extends User&lt;Principal&gt;</code>. Since <code>MyUser</code> deoesn't extend a <code>User&lt;Principal&gt;</code>, but a <code>User&lt;MyPrincipal&gt;</code>, it won't work as you expected. A <code>User&lt;MyPrincipal&gt;</code> is not a <code>User&lt;Principal&gt;</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&lt;P extends Principal, U extends User&lt;P&gt;&gt; { } </code></pre> <p>and now, you can create your <code>MyAction</code> like this:</p> <pre><code>public class MyAction extends Action&lt;MyPrincipal, MyUser&gt;{ } </code></pre> <hr /> <p>Or you can also modify your class using wildcard bound:</p> <pre><code>public abstract class Action&lt;U extends User&lt;? extends Principal&gt;&gt; { } </code></pre> <p>This way you won't need another type parameter:</p> <pre><code>public class MyAction extends Action&lt;MyUser&gt;{ } </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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    1. USRohit Jain
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    1. COYes, both option work. But my question is, why I can not define it as Action<U extends User<P extends Principal>>
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      1. USJose L Martinez-Avial
    2. CO@JoseLMartinez-Avial B'coz you haven't declared the type parameter `P`. A type parameter is said to be declared for a class if it comes at the top-level, and not when nested. And before using a type parameter for bounds, you must declare itself.
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