Guice CreationException - No implementation for type Foo was bound

Need some help to understand this error and possible way to solve it.

So I have a Type like this:

class Foo {
   static final class Bar {}

   @Inject
   Foo(TypeA a, @Nullable Set<Bar> b) {}
}

In order to make Foo injectable, I need to fulfill dependency for params a and b, now I have no problem to provide Type a implementation, its not null, so I have a @Provides method in the packages Module class. But what about b? Its a nullable, how to write @Provides with a parameter that can be nullable? My guess is it shouldn't be required and Guice should understand the annotation... But something isn't working right, I got this error:

1) No implementation for java.util.Set<Foo$Bar> was bound.

And here is the @Provides method (which didn't work atm)

@Provides @Nullable
public Foo provideFoo(TypeA a, @Nullable Set<Foo.Bar> b) {
    return new Foo(a, b);
}

I know the nested class is not public nor the constructor, so maybe guice can't access it but even if I make them all public (which I don't really want) the error still remains... Is there something I am m开发者_开发百科issing?

There's a difference between not having any binding for Set<Bar> and having a binding for Set<Bar> that is null.

You'd need an @Provides method that returns a Set<Bar> (that may be null) or something like bind(Bar.class).toProvider(Providers.ofInstance(null)) in order to be able to provide the second parameter to Foo's constructor.

Also note that your @Provides method for Foo is just calling the @Inject annotated constructor directly, so it's not necessary at all. I'd also say it'd be preferable to bind to an empty Set<Bar> rather than to null.