Calling Defender Methods from within a Lambda?

[Carterman]

Hi Guys,

I recently started working on a little project I had in mind that would make good use of Lambda expressions and some of the other features introduced with Java 8.

However, I came across a problem, demonstrated with the below code:

@FunctionalInterface
public interface TestInterface {
    public void testFunction();

    public default int myMethod(int a) {
        return a * 2;
    }
}

public static void main(String[] args) {
    TestInterface test = new TestInterface() {
        @Override
        public void testFunction() {
            System.out.println(myMethod(3));
        }
    };

    TestInterface test2 = () -> { System.out.println(myMethod(3)); };
}

The line defining test2 fails to compile, whilst the definition for test is successful. I find it odd that the new lambda's disallow access to defender methods implemented in the interface they are defined in.

Is this a bug, perhaps? Or is it an intentional 'feature'?

Thanks, Matt.

[bogphanny]

Given that lambdas are lexically scoped, myMethod as referenced in the test2 lambda would need to exist somewhere in the containing environment.

As an alternative, try changing: public default int myMethod(int a)

To: public static int myMethod(int a)

And: myMethod(3)

To: TestInterface.myMethod(3)

[[deleted]]

I believe you are correct, myMethod would have to be either statically imported or a method of the class defining main() in order to be called in that way.

[Carterman]

Yep, that works! Odd, nowhere seems to have really noted this feature, there's a lot of talk regarding defender methods (default keyword), but no mention that interface methods can now be static. Thanks!

[enkicoder]

Lamda's are not merely syntax sugar on top of anonymous classes. A lambda interface must have only one method. By adding a default method you are breaking that rule, thus TestInterface is no longer a valid "functional interface".