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POJava field type for a value of a generically recursive self-type?
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copied!<p>Given a class hierarchy where the base class defines a recursive self-type:</p> <pre><code>abstract class A<T extends A<T>> { } </code></pre> <p>How can I declare another class (which should not be generic in T, because such a T could vary over the lifetime of the object) with a field that can hold any subclass of A?</p> <p>The following does not work:</p> <pre><code>public class B { //fails to compile, because the capture of ? is not sufficiently narrow private A<?> a; public <T extends A<T>> setA(T a) { this.a = a; } } </code></pre> <p>-- END OF QUESTION --</p> <p>I've noticed a tendency of a number of StackOverflow members to approach certain difficult questions with "why are you doing that in the first place?" The following is a justification of my use of this pattern - you can note that the Java standard library also uses recursive self-types in its definition of the Enum class: <code>Enum<E extends Enum<E>></code>. This question could similarly be asked as "how to define a field of type <code>Enum<?></code>.</p> <p>Justification example:</p> <pre><code>abstract class A<T extends A<T>> { public abtract T self(); public B<T> bify(Bifyer bifyer) { return bifyer.bify(self()); } } </code></pre> <p>with subclasses:</p> <pre><code>class ASub1 extends A<ASub1> { public ASub1 self() { return this; } } class ASub2 extends A<ASub2> { public ASub2 self() { return this; } } </code></pre> <p>bound to a parallel class hierarchy:</p> <pre><code>abstract class B<T extends A<T>> { } class BSub1<T extends A<T>> implements B<T> { } class BSub2<T extends A<T>> implements B<T> { } //and others </code></pre> <p>And with generation of B instances managed by implementations of a Bifyer interface:</p> <pre><code>interface Bifyer { B<ASub1> bify(ASub1 asub1); B<ASub2> bify(ASub2 asub2); } </code></pre> <p>Implementations of this interface may return a BSub1 or BSub2 for the B. This is essentially an application of the Visitor pattern where the Bifyer is the visitor, but unlike the standard Visitor the accept method returns a value instead of void. This provides a modular framework where different Bifyer implementations can be specified to provide alternate behavior and return types for the Bify method - say one for each subclass of B.</p>

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