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PONested Type Parameters in Java
 

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    <p>This is an example which I made up to be a simplification of my real code, so I apologize if it is a little contrived. What I would like to do is to effectively get two type parameters out of a single nested type argument. I'm pretty sure this is impossible, but I thought I'd give it a shot.</p> <pre><code>//Not legal java code public class Foo&lt;C extends Collection&lt;T&gt;&gt; { //where T is another type parameter private C coll; public Foo(C coll) { this.coll = coll; } public void add(T elem){ this.coll.add(elem); } //UPDATED TO ADD GETTER /** * I may need to retrieve the collection again, or pass it * on to another function that needs the specific C type */ public C getColl(){ return coll; } } ... List&lt;String&gt; strings = new ArrayList&lt;String&gt;(); Foo&lt;List&lt;String&gt;&gt; foo = new Foo&lt;List&lt;String&gt;&gt;(strings); foo.add("hello"); </code></pre> <p>I know that I could do it by adding another type parameter:</p> <pre><code>public class Foo&lt;C extends Collection&lt;T&gt;,T&gt; </code></pre> <p>but then I have to add the redundant:</p> <pre><code>Foo&lt;List&lt;String&gt;,String&gt; foo = new Foo&lt;List&lt;String&gt;,String&gt;(strings); </code></pre> <p>And in my real world case, my generics can sometimes be specified in the implements clause like</p> <pre><code>public class Bar implements Baz&lt;String&gt; </code></pre> <p>Having to specify that second type parameter is even more painful then, because it feels like it throws the implementation details in my face. Having to say</p> <pre><code>Foo&lt;Bar,String&gt; </code></pre> <p>when there is a relationship between String and Bar already, just seems inelegant. I get that its Java, so that goes with the territory, but just curious if there was a solution for this.</p>
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