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    copied!<p>You're tripping over the fact that Java generics are not polymorphic on the type parameter.</p> <p>Talking through your code fragment, let's pull the example apart:</p> <pre><code>protected List&lt;GreyHound&gt; greyHounds; // List&lt;GreyHound&gt; is fine /** This method returns a lovely List of GreyHounds */ public List&lt;GreyHound&gt; getGreyHounds() { return this.greyHounds; } /** Here is the problem. A List&lt;GreyHound&gt; is not a List&lt;Dog&gt; */ public List&lt;Dog&gt; getDogs() { return getGreyHounds(); //compiler error } </code></pre> <p>So your original comment is correct. The two Lists are definitely different with no inheritance between them. So, I would suggest that you investigate these two options:</p> <ol> <li><p>Try returning a new list as you suggest in your comment. For example, <code>return new ArrayList&lt;Dog&gt;(this.greyHounds);</code></p></li> <li><p>Do you really need to keep a list of a specific breed of Dog? Perhaps you should define the data member to be a <code>List&lt;Dog&gt;</code> to which you add your specific GreyHounds. I.e., <code>protected List&lt;Dog&gt; greyHoundsOnly;</code> where you manage which dogs are allowed in the kennel via the object's external interface.</p></li> </ol> <p>Unless you have a good reason to keep a type-specific list, I would think seriously about option 2.</p> <p>EDIT: fleshing out my suggested options above:</p> <p>Option 1: <strong>Return a new list</strong>. Pros: Simple, straightforward, you get a typed list out and it eliminates a thread-safety problem (doesn't expose an internal reference to the world). Cons: seemingly a performance cost.</p> <pre><code>// Original code starts here. public interface DogKennel { public List&lt;Dog&gt; getDogs(); } public class GreyHoundKennel implements DogKennel { protected List&lt;GreyHound&gt; greyHounds; public List&lt;GreyHound&gt; getGreyHounds() { return this.greyHounds; } // Original code ends here public List&lt;Dog&gt; getDogs() { // This line eliminates the thread safety issue in returning // an internal reference. It does use additional memory + cost // CPU time required to copy the elements. Unless this list is // very large, it will be hard to notice this cost. return new ArrayList&lt;Dog&gt;(this.greyHounds); } } </code></pre> <p>Option 2: <strong>Use a different data representation</strong>. Pros: plays nicer with polymorphism, returns the generic list that was the original goal. Cons: it's a slightly different architecture which may not fit with the original task.</p> <pre><code>public abstract class DogKennel { protected List&lt;Dog&gt; dogs = new ArrayList&lt;Dog&gt;(); } public class GreyHoundKennel extends DogKennel { // Force an interface that only allows what I want to allow public void addDog(GreyHound greyHound) { dogs.add(greyHound); } public List&lt;Dog&gt; getDogs() { // Greatly reduces risk of side-effecting and thread safety issues // Plus, you get the generic list that you were hoping for return Collections.unmodifiableList(this.dogs); } } </code></pre>
 

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