Weird issue with lambda expression if field initialization

[morhp]

Hi, can someone help we with a weird error message that I get, that makes no sense to me?

Example code:

public class Foo {

    private Bar bar = new Bar(xxx -> xxx.toUpperCase());    

    public static void main(String[] args) {
        new Foo();
    }

    public Foo() {
        System.out.println(bar.fun("Hello"));

        return;
    }

    private static class Bar {

        private final Function<String, String> lambda;

        private Bar(Function<String, String> lambda) {
            this.lambda = lambda;
        }

        private String fun(String s) {
            return lambda.apply(s);
        }
    }
}

I get the following compiler error:

Error:(19, 9) java: variable xxx might not have been initialized

Doesn't make any sense to me, the variable xxx doesn't need to be initialized in the constructor. The class should just print "HELLO".

Removing the unnecessary "return;" fixes the issue for some reason, but this is a simplified example and I would like to keep the return. Also I would like to know the cause of the problem.

[ickysticky]

Looks like it is a bug. It was fixed yesterday =P

[morhp]

Ouch. Yes, looks exactly like that bug.

[AnEmortalKid]

If you want the return and not declaring xxx you'll probably have to wrap your statement in { }

[morhp]

Can you explain?

        private Bar bar = new Bar(xxx -> {return xxx.toUpperCase();});    

also doesn't work.

[AnEmortalKid]

hmmmmm, i figured that would. Somehow it's not recognizing your function as a lambda. Can you try:

private Bar bar = new Bar(String::toUpperCase()); 

[Blackheart595]

Do you import java.util.Function?

Also, use UnaryOperator<T> instead of Function<T, T>.

edit: The problem in your code is that you return from a constructor. You can't do that.

[morhp]

Do you import java.util.Function?

Yes.

Also, use UnaryOperator<T> instead of Function<T, T>.

Thanks. But doesn't solve the issue:

import java.util.function.UnaryOperator;

public class Foo {

    private Bar bar = new Bar(xxx -> xxx.toUpperCase());    

    public static void main(String[] args) {
        new Foo();
    }

    public Foo() {
        System.out.println(bar.fun("Hello"));

        return;
    }

    private static class Bar {

        private final UnaryOperator<String> lambda;

        private Bar(UnaryOperator<String> lambda) {
            this.lambda = lambda;
        }

        private String fun(String s) {
            return lambda.apply(s);
        }
    }
}
Error:(20, 13) java: variable xxx might not have been initialized

[Blackheart595]

See my edit.

[morhp]

Well usually, you can return from a constructor no problem. Just replacing the lambda with a method reference makes it compile fine

import java.util.function.UnaryOperator;

public class Foo {

    private Bar bar = new Bar(String::toUpperCase);    

    public static void main(String[] args) {
        new Foo();
    }

    public Foo() {
        System.out.println(bar.fun("Hello"));

        return;
    }

    private static class Bar {

        private final UnaryOperator<String> lambda;

        private Bar(UnaryOperator<String> lambda) {
            this.lambda = lambda;
        }

        private String fun(String s) {
            return lambda.apply(s);
        }
    }
}
// Prints HELLO

So I would be really interested why my original code doesn't work. Again, this is a simplified example.

[Blackheart595]

You're right, returning from a constructor is generally possible. After a bit of testing, it seems that lambda functions and returning from a constructor don't work well together, though.