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POImplement Comparable<T> in a generic superclass
 

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    <p>I have an abstract generic superclass which I extend to make concrete classes with IDs of a specific type:</p> <pre><code>public abstract class AbstractEntity&lt;I&gt; { ... private I id; private String name; ... } </code></pre> <p>I create the concrete classes like this:</p> <pre><code>public class User extends AbstractEntity&lt;Long&gt; {...} </code></pre> <p>Now I want to make the concrete classes comparable to other instances of themselves (and no other type--this should be enforced at compile time). By default the <code>compareTo</code> method should just sort by name so I want to add it in the superclass to avoid repeating code. The only way I've come up with to do this is with this ugly construct:</p> <pre><code>public abstract class AbstractEntity&lt;I,T&gt; implements Comparable&lt;AbstractEntity&lt;I,T&gt;&gt; { ... private I id; private String name; ... @Override public int compareTo(AbstractEntity&lt;I,T&gt; o) { ... } } </code></pre> <p>...</p> <pre><code>public class User extends AbstractEntity&lt;Long,User&gt; {...} </code></pre> <p>I think this is ugly because now in order to extend the superclass you need to repeat the concrete class name again as a type parameter for itself, which is odd and counterintuitive. Is there a more elegant way to accomplish the same thing that I'm missing?</p> <p>Thanks!</p>
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    1. COTechnically, one could "break the implied contract" and create a concrete class which was comparable to some other class with something like "class Foo extends AbstractEntity<Long,User>", but I'm not that worried about cases where people go out of their way to write garbage.
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    2. COIs there any reason you must use `Comparable` rather than `Comparator`? The `Comparator` method seems more suitable for use with inheritable types. If you will need to be able to compare items without having a `Comparator` handy, but you will control the types in question, it might also be helpful to define an interface which allows an object to be asked for a `Comparator` that is suitable for use with it. If one has a `Comparator` that can handle all items in an array, one can cleanly sort the array even if the items are of different types.
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    3. CO@supercat - Mainly because it would require fewer client-side changes to reap the benefits. If I used a Comparator, the client code would have to acquire the Comparator and explicitly use it (either by creating a collection that uses it or by passing it to a sort function). They would have to do this everywhere they want to sort. This way, if the client code happens to place the objects of this type in a naturally sorted Collection (or they call a natural sort function), it will just start working.
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