Why is this type inference not working with this Lambda expression scenario?

Question

I have a weird scenario where type inference isn\'t working as I\'d expect when using a lambda expression. Here\'s an approximation of my real scenario:

stat         


        

Answer 1

I don't know why but you need to add separate return type:

public class HelloWorld{
static class Value {
}

@FunctionalInterface
interface Bar {
      R apply(Value value); // Return type added
}

static class Foo {
  public static  R foo(Bar callback) {
      return callback.apply(new Value());
  }
}

void test() {
  System.out.println( Foo.foo(value -> true).booleanValue() ); // No compile error here
}
     public static void main(String []args){
         new HelloWorld().test();
     }
}

some smart guy probably can explain that.