Self bound generic type with fluent interface and inheritance

Question

I am using a fluent interface with inheritance. I declared the base class Constructor protected so you cant create a Foo<Bar> which would result in a ClassCastException on calling add(). But i am having trouble with the static method that returns a new Foo instance.

public class Foo<T extends Foo<T>> // if i change to extends Foo i only get warnings { public static Foo<Foo> createFoo() // <-- error { return new Foo<Foo>(); // <-- error } protected Foo() {} public T add() { //... return (T)this; } } public class Bar extends Foo<Bar> { public Bar sub() { //... return this; } }

This is mostly an excercise(personal not homework) in Fluent Interfaces, Domain-specific language and Generics, so please dont ask what i need it for.

Edit: Eclipse error

Bound mismatch: The type Foo is not a valid substitute for the bounded parameter <T extends Foo<T>> of the type Foo<T>

This answer should help explain the meaning of the error you're seeing (after the bold EDIT specifically). In the future please post the complete error message in your question. — Paul Bellora
– Paul Bellora, Commented Oct 31, 2011 at 20:46
I dont know what the downvote is for, i searched for selfbound generic types on google and could not find an answer to my problem. — Stefan
– Stefan, Commented Oct 31, 2011 at 21:03
The answer linked actually helped me solve the problem. Change Foo declaration to: public abstract class AbstractFoo<T extends AbstractFoo<T>>. Create final class ConcreteFoo extends AbstractFoo<ConcreteFoo> without extra functionality. It does however destroy some of the inheritance. — Stefan
– Stefan, Commented Oct 31, 2011 at 21:10
Generics can really be a pain if you "misuse" (no offense) them like this. I am really a friend of them, but sometimes they make it much harder to write code as without them. Try to declare different types instead of a recursive structure, which is hard to maintain and extend. — Stephan
– Stephan, Commented Oct 31, 2011 at 21:11

Mark Peters · Accepted Answer · 2011-10-31 21:18:34Z

You essentially have a recursive type declaration.

Foo<T extends Foo<T>>.

So let's say you have a Foo<Foo>. That means T is mapped to Foo. But Foo is not subtype of Foo<T>, which in this case is Foo<Foo>, so what you're really looking for is Foo<Foo<Foo>>. But wait a minute, the innermost Foo isn't typed, so I guess it's Foo<Foo<Foo<Foo>>>...oh forget it!

To put a more understandable face on it, consider if you had Foo<T extends List<T>>. What could you possibly use for T in a declaration/instantiation of Foo? List<String>? List<List>?

Edit

It looks like you found a way to "break" the recursion cycle. You eventually need to get to a reified type. In the same way that you found that ConcreteFoo worked for you, you could similarly for the List example above have some class ConreteListOfItself implements List<ConreteListOfItself> that would break the recursion cycle.

I guess the problem stems from the fact that Foo doesnt extend itself.
@Stefan: Well, extends in the context of a type bound means a subclass or the class itself. So the problem isn't that Foo doesn't extend Foo, the problem is that Foo doesn't extend Foo<Foo>. Just like List doesn't extend List<List>, but some subclass could.
I am going to accept this answer, as it explains why what im trying to do 'doesnt' work. At this point i think i need a more powerful Generics mechanism in line of what Kublai Khan says here in order to get working fluent interfaces with inheritance structure.

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