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POInstantiating protected class from outside package
 

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    <p>I have rewritten the Binary Search Tree code found in the following links to work generically - the class will work with any type parameter. <a href="http://www.informatics.susx.ac.uk/courses/dats/DataStructures/SearchTree.java" rel="nofollow">http://www.informatics.susx.ac.uk/courses/dats/DataStructures/SearchTree.java</a> http://www.informatics.susx.ac.uk/courses/dats/teach/code/demos/SearchTreeDemo.java</p> <p>I placed my generic SearchTree.java in a package named mysearchtree, and imported that into SearchTreeDemo.java.</p> <p>SearchTree.java contains: Abstract class SearchTree. Concrete class EmptyTree, which has a protected constructor. Concrete class NodeTree, which also has a protected constructor.</p> <p>The original, non-generic coding for the SearchTree abstract class was that a client of the code would instantiate an EmptyTree object through a static method as below:</p> <pre><code>public abstract class SearchTree { /** * Returns an empty tree. */ public static SearchTree empty() { return new EmptyTree(); } // more code } </code></pre> <p>This would have been used as follows:</p> <pre><code>SearchTree t = SearchTree.empty(); t = t.add( new String("hello") ); </code></pre> <p>When working with generics, I have re-written the class to have the following structure:</p> <pre><code>public abstract class SearchTree&lt;T extends Comparable&lt;T&gt;&gt; { public abstract boolean isEmpty(); public abstract int numNodes(); public abstract boolean contains(T key); public abstract SearchTree&lt;T&gt; add(T item); public abstract SearchTree&lt;T&gt; remove(T item); public abstract String toString(); } </code></pre> <p>The original plan of providing a static method that would return an EmptyTree will not work generically (you can't use the generic type parameters of the class in a static method).</p> <p>My question is: How do I provide a type-safe way of allowing a client of this code to instantiate a EmptyTree from outside the package (or some other equivalent mechanism that would replace the <code>SearchTree t = SearchTree.empty()</code> code)?</p> <p><strong>Update:</strong></p> <p>I have tried the following, but get an error:</p> <pre><code>public static &lt;U&gt; SearchTree&lt;U&gt; createSearchTree() { return new EmptyTree&lt;U&gt;(); } </code></pre> <p>Errors:</p> <pre><code>./mysearchtree/SearchTree.java:7: type parameter U is not within its bound public static &lt;U&gt; SearchTree&lt;U&gt; createSearchTree() { return new EmptyTree&lt;U&gt;(); } ^ ./mysearchtree/SearchTree.java:7: type parameter U is not within its bound public static &lt;U&gt; SearchTree&lt;U&gt; createSearchTree() { return new EmptyTree&lt;U&gt;(); } ^ 2 errors </code></pre>
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