Type inference with mutually recursive generic types

Niklas Matthies <usenet-nospam@nmhq.net>
24 Oct 2006 13:58:35 GMT
It is possible to define mutually recursive generic types like:

    interface Parent<P extends Parent<P, C>, C extends Child<P, C>>
        Set<C> children();

    interface Child<P extends Parent<P, C>, C extends Child<P, C>>
        P parent();

Now one can write (for example):

    class HumanParent implements Parent<HumanParent, HumanChild>
        public Set<HumanChild> children() { ... }

    class HumanChild implements Child<HumanParent, HumanChild>
        public HumanParent parent() { ... }

But now I need to write a method (in some other class) like:

    static <P extends Parent<P, C>, C extends Child<P, C>>
       C someChildOf(P parent)
        return parent.children().iterator().next();

I can use it with concrete types:

    HumanParent humanParent = ...;
    HumanChild humanChild = someChildOf(humanParent);

But when I try to use it with the generic wildcard type

    Parent<?, ?> parent = ...;
    Child<?, ?> child = someChildOf(parent); // doesn't compile

then javac complains:

   <P,C>someChildOf(P) [...] cannot be applied to
      (Parent<capture of ?,capture of ?>)

Is there any way to get type parameters to bind for recursive types
like Parent<?, ?> and Child<?, ?>?

Incidentally you already get the same error with just one type

    static <E extends Enum<E>> E f(E e) { ... }

    Enum<?> e = null;
    f(e); // doesn't compile:

   <E>f(E) [...] cannot be applied to (java.lang.Enum<capture of ?>)

Of course in this case one can define f() as

    static <E extends Enum<?>> E f(E e) { ... }

which compiles fine, but that's not an option with Parent/Child
because it breaks the Parent<->Child type mapping:

    static <P extends Parent<?, ?>, C extends Child<?, ?>>
       C someChildOf(P parent)
        return parent.children().iterator().next(); // doesn't compile

    incompatible types
    found : Child<P,C>
    required: C

Variations like

    static <P extends Parent<?, C>, C extends Child<P, ?>>
       C someChildOf(P parent) // doesn't compile
        return parent.children().iterator().next();

    type parameter C is not within its bound
    type parameter P is not within its bound

obviously don't work either.

Any suggestions?

-- Niklas Matthies

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