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    <p>In Java, every generic type must be specified. You can go without specifying any type, but you can't go without specifying just one. </p> <p>Also, <strong>every</strong> generic type must be specified in the declaration. If you want to have <code>class GenericDAO&lt;T extends Identifable&lt;U&gt;&gt;</code>, you must add the generic type declaration for <code>U</code> to your class declaration like this (since <code>U</code> is actually a generic type here): </p> <pre><code>public abstract class GenericDAO&lt;T extends Identifable&lt;U&gt;, U&gt; </code></pre> <p>The following is partially off-topic, but you might find it useful.</p> <p>I've noticed that in your definition of <code>GenericDAO</code> two generic types are not tied to each other. This might not be what you want.</p> <p>What you have here is a particular case in which the two generics are matching (the <code>Long</code> type in the <code>Cat</code> and <code>CatDAO</code> definitions). Consider having these declarations:</p> <pre><code>public class Dog implements Identifable&lt;Long&gt; public class DogDAO extends GenericDao&lt;Dog, String&gt; </code></pre> <p>This would force you to write the <code>getById</code> method in <code>DogDAO</code> method:</p> <pre><code>Dog getById(String id); </code></pre> <p>Your <code>getId</code> method in the <code>Dog</code> returns a <code>Long</code> so your <code>getById</code> method int <code>DogDAO</code> would have to compare <code>String</code>s to <code>Long</code>s. This is valid thing to do, but it's a bit counter-intuitive. Having a <code>getById</code> method for <code>DogDAO</code> that takes a <code>Long</code> parameter makes more sense, since the <code>Dog</code>s IDs are actually <code>Long</code>s.</p> <p>If you want to tie the two types together, you can define the <code>GenericDAO</code> class as:</p> <pre><code>public abstract class GenericDAO&lt;T extends Identifable&lt;S&gt;, S&gt; </code></pre> <p>You still have to specify the second parameter, but at least the compiler can help you make sure that the types are matching.</p>
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    1. COthanks for explanation. and for off-topic :)
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