Feedback and comments on ARM proposal (original) (raw)

Neal Gafter neal at gafter.com
Wed Mar 18 08🔞24 PDT 2009

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.

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