Feedback and comments on ARM proposal - resend

[Neal Gafter]

On Wed, Mar 18, 2009 at 1:15 AM, Jeremy Manson <jeremy.manson at gmail.com> wrote:
> I'm pretty sure that it is only definitely unassigned there if it is
> definitely unassigned after the switch expression.  If you want it to
> compile, you have to place the declaration before the switch
> statement.  I could be wrong about that.

JLS 16.2.2 says

# A local variable V is definitely unassigned (and moreover is not
definitely assigned) before the block that is the body of the
constructor, method, instance initializer or static initializer that
declares V .

So moving the declaration up should have no impact.  And yet the
following program compiles.  Why should this compile and the previous
one fail to compile?

import java.io.*;

class T {
    public static void main(String[] args) {
	switch (args.length) {
	case 0:
	    final int i;
	    i = 0;
	    break;
	case 1:
	    i = 1; // i is definitely unassigned here, so we assign to it
	    System.out.println(i);
	    break;
	}
    }
}

spoiler below.






The answer can be found in JLS 15.26:  "A variable that is declared
final cannot be assigned to (unless it is a definitely unassigned
(§16) blank final variable (§4.12.4)), ..."

Note particularly the word "blank".  You're not allowed to assign to a
final variable that is definitely unassigned unless it is a blank
final.  The original version of this puzzler is the only way to create
a variable that is final and definitely unassigned, but not a blank
final.  It also shows a way to create a constant variable that is not
definitely assigned.  Should the following snippet compile?

	switch (args.length) {
	case 0:
	    final int i = 0; // declare a constant
	    break;
	case 1:
	    System.out.println(i); // print the value of the constant variable
	    break;
	}

The point of this series of puzzles is that, due to the strangeness of
the switch statement, building a resource language construct based on
variable scope can be subtle.  The simplest way to avoid problems is
to say that you can't use a resource-variable-declaration-statement at
the top level within a switch statement.