allowable values for case labels ?

[Rodge Thomas]

FROM: www.danchisholm.net/august04/topic/control.html
(older exam version)

class Blue {
  public static void main(String args[]) {
    long[] l = {1, -1, 3, 4};
    for (int i = 0; i<l.length;i++) {
      switch(l[i]) {
        case 2-1: System.out.print("v ");
        break;
        case 'd'-'a': System.out.print("w ");
        break;
        case 0: System.out.print("x ");
        break;
        case ~0: System.out.print("y ");
        break;
        case 4&5: System.out.print("z ");
        break;
        default: System.out.print("Default ");
      }
    }
  }
}

What is the result of attempting to compile and run the above program?
a. Prints: v w x y
b. Prints: v w x y z Default
c. Prints: v y w z
d. Prints: Default Default Default Default
e. Runtime Exception
f. Compiler Error
g. None of the Above
ANSWER: C
EXPLANATION:
The constant case expressions can be any expression that is assignable to the type of the switch expression. 'd'-'a'=3. ~0=-1. 4&5=4.
-----------
I thought that the rules for what is allowed as x for " case x: " are as follows:
(1) x must be assignable to the variable in the switch statement
(2) x can be byte, char, short, or int only
(3) According to Mughal, "case labels are constant expressions"
*this is where I am unsure*
When I think "constant expressions", I think of values that are known at compile time. Such as the number 7 or a final int variable j. But, 'd'-'a', ~0, 4&5 are computed at runtime, right?
any insight will be very much appreciated.
[ Jess added [URL=http://www.javaranch.com]UBB [CODE] tags ]
[ August 19, 2002: Message edited by: Jessica Sant ]

[Anthony Villanueva]

Compile time constant expressions can be used in case labels. See the JLS.

[Barkat Mardhani]

I think that compile time computable expressions
are also candidate for x in
case x
I think only limitation is that x should result
in an integral value. Anyone: correct me if
I am wrong?
Thanks
Barkat
[ August 19, 2002: Message edited by: Barkat Mardhani ]

[Jessica Sant]

�14.10 The switch Statement
"The type of the Expression must be char, byte, short, or int, or a compile-time error occurs."

[Barkat Mardhani]

Hi Jesica:
I think it is difference in term of reference. Bruce Eckel (Thinking in Java) is saying that
"expression should result in an integral value".
The types in your quote will all result in integral values including char. If there is any
other type that results in integral value, then
we have a confict...
Thanks
Barkat

[maheshkumar]

Program is giving compiler time error:
" possible loss of precision " at line
switch(l[i])
instead of that if i use
switch((int)l[i])
Then it is giving the output correctly

[Venkatesh Gowri]

Hi
I guess the switch label should be compatible to an integral value.
this is what RHE States.please correct me if I am wrong
thanks

[Valentin Crettaz]

mahesh2002,
Welcome to Javaranch
We'd like you to read the Javaranch Naming Policy and change your publicly displayed name (change it here) to comply with our unique rule. Thank you.

[Dan Chisholm]

My mock exam has two questions that are very similar.

class Green {
  public static void main(String args[]) {
    short[] s = {1, -1, 3, 4};
    for (int i = 0; i<s.length;i++) {
      switch(s[i]) {
        case 2-1: System.out.print("v ");
        break;
        case 'd'-'a': System.out.print("w ");
        break;
        case 0: System.out.print("x ");
        break;
        case ~0: System.out.print("y ");
        break;
        case 4&5: System.out.print("z ");
        break;
        default: System.out.print("Default ");
      }
    }
  }
}

Result: Prints: v y w z
Remark:
The constant case expressions can be any expression that is assignable to the type of the switch expression. 'd'-'a'=3. ~0=-1. 4&5=4.

class Blue {
  public static void main(String args[]) {
    long[] l = {1, -1, 3, 4};
    for (int i = 0; i<l.length;i++) {
      switch(l[i]) {
        case 2-1: System.out.print("v ");
        break;
        case 'd'-'a': System.out.print("w ");
        break;
        case 0: System.out.print("x ");
        break;
        case ~0: System.out.print("y ");
        break;
        case 4&5: System.out.print("z ");
        break;
        default: System.out.print("Default ");
      }
    }
  }
}

Result: Compiler Error
Remark:
The switch expression can not be of type long. The legal types for the switch expression are byte, short, char, and int.
The question and answer that started this thread don't agree because the second question was paired with the first answer.