问题
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.
回答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;
}
}
来源:https://stackoverflow.com/questions/695298/java-field-type-for-a-value-of-a-generically-recursive-self-type