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POGeneric Restriction Hell: Bound Mismatch
 

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    <p>I'm working on a project that has an extensive tree of generic inheritance and dependencies. Go to edit to see better example. <strike>The basics look something like this:</p> <pre><code>class A { ... } class B { ... } class C extends B { ... } class D&lt;T extends B&gt; extends A { ... } class StringMap&lt;T extends A&gt; { HashMap&lt;String, T&gt; _elements; ... } </code></pre> <p>So now I'm going to write a class that contains a specific <code>StringMap</code> type.</p> <pre><code>class X { StringMap&lt;D&lt;C&gt;&gt; _thing = new StringMap&lt;D&lt;C&gt;&gt;; ... } </code></pre> <p>So far this all works fine. <code>D&lt;C&gt;</code> is actually a very long name and the specific combination is going to show up very frequently in other parts of the code, so I decided to a class for the specific combination so it will be clearer and have a shorter name.</p> <pre><code>class DC extends D&lt;C&gt; { } //and go to update X class X { StringMap&lt;D&lt;C&gt;&gt; _thing = new StringMap&lt;D&lt;C&gt;&gt;(); //still works fine StringMap&lt;DC&gt; _thing = new StringMap&lt;DC&gt;(); //error ... } </code></pre> <p></strike></p> <p>Eclipse gives the error of</p> <blockquote> <p>Bound mismatch: The type <code>DC</code> is not a valid substitute for the bounded parameter <code>&lt;T extends A&gt;</code> of the type <code>StringMap&lt;T&gt;</code></p> </blockquote> <p>So the question is, why does this not just work? <code>DC</code> does nothing but extend <code>D&lt;C&gt;</code> and echo the constructors. Why does <code>StringMap</code> see <code>DC</code> as different when it is just a child class of something it excepts?</p> <p><strong>EDIT:</strong><br> OK, reworked the example to be closer to what I'm actually doing. I tested it and it does produce the error. What I'm doing here is using the generic type to ensure that <code>clone()</code> returns the correct class for whoever implements it down the inheritance tree. Then in subclasses, I'm using <code>B&lt;T extends B&lt;T&gt;&gt;</code> to ensure that subclasses of <code>B</code> are passing in a subclass of B as the generic type <code>T</code>.</p> <pre><code>public abstract class Undoable&lt;T&gt; implements Comparable&lt;T&gt; { public abstract T clone(); public abstract void updateFields(T modified); } abstract public class A&lt;T extends A&lt;T, U&gt;, U extends Comparable&lt;U&gt;&gt; extends Undoable&lt;T&gt; { abstract U getKey(); @Override public int compareTo(T element) { return getKey().compareTo(element.getKey()); } } public class B&lt;T extends B&lt;T&gt;&gt; extends A&lt;T, String&gt; { @Override public T clone() { // TODO Auto-generated method stub return null; } @Override public void updateFields(T modified) { // TODO Auto-generated method stub } @Override String getKey() { // TODO Auto-generated method stub return null; } } public class C extends B&lt;C&gt; { } public class D&lt;T extends B&lt;T&gt;&gt; extends A&lt;D&lt;T&gt;, String&gt; { @Override String getKey() { // TODO Auto-generated method stub return null; } @Override public D&lt;T&gt; clone() { // TODO Auto-generated method stub return null; } @Override public void updateFields(D&lt;T&gt; modified) { // TODO Auto-generated method stub } } public class DC extends D&lt;C&gt; { } public class StringMap&lt;T extends Undoable&lt;T&gt;&gt; { HashMap&lt;String, T&gt; _elements; } public class Main { public static void main(String[] args) { StringMap&lt;D&lt;C&gt;&gt; _thing = new StringMap&lt;D&lt;C&gt;&gt;(); //works StringMap&lt;DC&gt; _thing1 = new StringMap&lt;DC&gt;(); //error //Bound mismatch: The type DC is not a valid substitute for //the bounded parameter &lt;T extends Undoable&lt;T&gt;&gt; of the type StringMap&lt;T&gt; } } </code></pre>
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    2. COEdited, that was just there for the sample code I wrote for the question.
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