Question

Given a class hierarchy where the base class defines a recursive self-type:

abstract class A<T extends A<T>> { } 

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?

The following does not work:

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;     } } 

— END OF QUESTION —

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: Enum<E extends Enum<E>>. This question could similarly be asked as ‘how to define a field of type Enum<?>.

Justification example:

abstract class A<T extends A<T>> {     public abtract T self();     public B<T> bify(Bifyer bifyer) {         return bifyer.bify(self());     } } 

with subclasses:

class ASub1 extends A<ASub1> {      public ASub1 self() { return this; } }         class ASub2 extends A<ASub2> {      public ASub2 self() { return this; } }        

bound to a parallel class hierarchy:

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 

And with generation of B instances managed by implementations of a Bifyer interface:

interface Bifyer {     B<ASub1> bify(ASub1 asub1);     B<ASub2> bify(ASub2 asub2);         } 

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.

Answer 1

If you bound the wildcard ? below by A, it should work:

public class B {     private A<? extends A> a;      public <T extends A<T>> void setA(T a) {         this.a = a;     } }