Declaring and initializing variables within Java switches

With the integration of JEP 325: Switch Expressions (Preview) in JDK-12 early access builds. There are certain changes that could be seen from Jon's answer -

1. Local Variable Scope - The local variables in the switch cases can now be local to the case itself instead of the entire switch block. An example (similar to what Jon had attempted syntactically as well) considering the Day enum class for further explanation :

   public enum Day {
       MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY, SATURDAY, SUNDAY
   }
   
   // some another method implementation
   Day day = Day.valueOf(scanner.next());
   switch (day) {
       case MONDAY,TUESDAY -> {
           var temp = "mon-tue";
           System.out.println(temp);
       }
       case WEDNESDAY,THURSDAY -> {
           var temp = Date.from(Instant.now()); // same variable name 'temp'
           System.out.println(temp);
       }
       default ->{
           var temp = 0.04; // different types as well (not mandatory ofcourse)
           System.out.println(temp);
       }
   }
   

2. Switch Expressions - If the intent is to assign a value to a variable and then make use of it, once can make use of the switch expressions. e.g.

   private static void useSwitchExpression() {
       int key = 2;
       int value = switch (key) {
           case 1 ->  1;
           case 2 -> 2;
           default -> {break 0;}
       };
       System.out.println("value = " + value); // prints 'value = 2'
   }
   

The variable has been declared (as an int), but not initialized (assigned an initial value). Think of the line:

int value = 1;

As:

int value;
value = 1;

The int value part tells the compiler at compile time that you have a variable called value which is an int. The value = 1 part initializes it, but that happens at run-time, and doesn't happen at all if that branch of the switch isn't entered.

From http://www.coderanch.com/t/447381/java-programmer-SCJP/certification/variable-initialization-within-case-block

Declarations are processed at compile time and do not depend on the execution flow of your code. Since value is declared within the local scope of the switch block, it is useable anywhere in that block from the point of its declaration.

Switch statements are odd in terms of scoping, basically. From section 6.3 of the JLS:

The scope of a local variable declaration in a block (§14.4) is the rest of the block in which the declaration appears, starting with its own initializer and including any further declarators to the right in the local variable declaration statement.

In your case, case 2 is in the same block as case 1 and appears after it, even though case 1 will never execute... so the local variable is in scope and available for writing despite you logically never "executing" the declaration. (A declaration isn't really "executable" although initialization is.)

If you comment out the value = 2; assignment, the compiler still knows which variable you're referring to, but you won't have gone through any execution path which assigns it a value, which is why you get an error as you would when you try to read any other not-definitely-assigned local variable.

I would strongly recommend you not to use local variables declared in other cases - it leads to highly confusing code, as you've seen. When I introduce local variables in switch statements (which I try to do rarely - cases should be very short, ideally) I usually prefer to introduce a new scope:

case 1: {
    int value = 1;
    ...
    break;
}
case 2: {
    int value = 2;
    ...
    break;
}

I believe this is clearer.