The following code is an example of a factory that produces a `Bar<T>`
given a `Foo<T>`. The factory doesn't care what `T` is: for any type
`T`, it can make a `Bar<T>` from a `Foo<T>`.
import com.google.inject.*;
import com.google.inject.assistedinject.*;
class Foo<T> {
public void flip(T x) { System.out.println("flip: " + x); }
}
interface Bar<T> {
void flipflop(T x);
}
class BarImpl<T> implements Bar<T> {
Foo<T> foo;
@Inject
BarImpl(Foo<T> foo) { this.foo = foo; }
public void flipflop(T x) { foo.flip(x);
System.out.println("flop: " + x); }
}
interface BarFactory {
<T> Bar<T> create(Foo<T> f);
}
class Module extends AbstractModule {
public void configure() {
bind(BarFactory.class)
.toProvider(
FactoryProvider.newFactory( BarFactory.class,
BarImpl.class )
);
}
}
public class GenericInject {
public static void main(String[] args) {
Injector injector = Guice.createInjector(new Module());
Foo<Integer> foo = new Foo<Integer>();
Bar<Integer> bar =
injector.getInstance(BarFactory.class).create(foo);
bar.flipflop(0);
}
}
When I run the code, I get the following errors from Guice:
1) No implementation for BarFactory was bound.
at Module.configure(GenericInject.java:38)
2) Bar<T> cannot be used as a key; It is not fully specified.
I guess the problem is that the BarFactory.create() method doesn't
match the pattern expected by FactoryProvider, because of the type
quantifier <T>. But there isn't a real problem here: BarImpl.class
(with no type literal) is exactly what we need to create a Bar<T>
given a Foo<T>. Is there a way to configure Guice to generate a
BarFactory instance for me, or do I just need to write the BarFactory
instance by hand?
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