Java 8 lambda generic interface method

Question

@FunctionalInterface
public interface GenericFunctionalInterface {
  public <T> T genericMethod();
}

I have above @FunctionalInterface and it has a generic method.

How can I use and Lambda expression to represent this Interface?

I tried below code, but it doesn't work,

GenericFunctionalInterface gfi = () -> {return "sss";};

I got compile error: Illegal lambda expression: Method genericMethod of type GenericFunctionalInterface is generic

Where can I place the type info?

See this answer and section §15.27.3 in JLS8. "A lambda expression is congruent with a function type if all of the following are true: The function type has no type parameters." — McDowell
– McDowell, Commented Jun 30, 2016 at 9:41
Do you mean “gener̲ic” method? I don’t see any connection to Genetics. Besides that, how is that supposed to work? The method <T> T geneticMethod() promises to return whatever the caller wishes, so a lambda expression return a String doesn’t fulfill that. — Holger
– Holger, Commented Jul 4, 2016 at 16:32
Thanks Holger. it's generic. I'm wondering whether I can set the type info for the lambda expression. — jack yin
– jack yin, Commented Jul 6, 2016 at 3:41

Eran · Accepted Answer · 2016-06-30 08:50:12Z

10

The generic (not genetic) type parameter should be declared in the interface level, not in the method level :

public interface GenericFunctionalInterface<T> {
  public T genericMethod();
}

GenericFunctionalInterface<String> gfi = () -> {return "sss";};

answered Jun 30, 2016 at 8:50

Eran

395k57 gold badges726 silver badges793 bronze badges

Sign up to request clarification or add additional context in comments.

1 Answer 1