Effectively final local variable cannot be declared as final

[Tagir Valeev]

Hello!

Consider the following code:

public class Demo {
    public static void main(String[] args) {
        for (int i = 0; i < 10; i++) {
            Runnable r = () -> System.out.println(i);
            r.run();
            break;
        }
    }
}

It can be compiled with javac (version 19), successfully executed and
prints 0. As only effectively final locals are allowed to be captured
in lambda, I conclude that `i` is effectively final. I also assumed
that for every effectively final variable, it's possible to write an
explicit `final` modifier. However, this is not the case. The
following code fails with the compilation error:

public class Demo {
    public static void main(String[] args) {
        for (final int i = 0; i < 10; i++) {
            Runnable r = () -> System.out.println(i);
            r.run();
            break;
        }
    }
}
Demo.java:3: error: cannot assign a value to final variable i
        for (final int i = 0; i < 10; i++) {
                                      ^
1 error
error: compilation failed

In my opinion, either both should be accepted or both should be
rejected. According to JLS 4.12.4, I'm inclined that both should be
rejected:

A local variable whose declarator has an initializer (§14.4.2) is
effectively final if all of the following are true:
- It is not declared final.
- It never occurs as the left hand side in an assignment expression
(§15.26). (Note that the local variable declarator containing the
initializer is not an assignment expression.)
- It never occurs as the operand of a prefix or postfix increment or
decrement operator (§15.14, §15.15).

Here, `i` has an initializer and also occurs as the operand of prefix
increment, so it cannot be effectively final.

What do you think?

With best regards,
Tagir Valeev.