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What is the "Compile time constant" norm for a switch case constant ?

843789Oct 30 2009 — edited Oct 30 2009
Hi Guys,

AS per the Kathy Sierra written book for SCJP 1.5 :

The case constant for a switch statement must be a compile time constant,only final is not enough.

Now,what is a compile time constant ?

Don't even integers which are declared normally ( For ex. int k=5;) act as case constants in a switch statement ?

Thanks.

Comments

masijade
No
case 5:  is a aconstant
case '0': is a constant
static final int k = 5;
case k:  is a constant
etc
843789
masijade. wrote:
No
case 5:  is a aconstant
case '0': is a constant
static final int k = 5;
case k:  is a constant
etc
My bad,not able to make out what you want to say.

Now what would not qualify as a case constant in this code :

package flowcontrol;

public class Fc1 {

	/**
	 * @param args
	 */
	public static void main(String[] args) {
		// TODO Auto-generated method stub
		int day=2;
		int year=2000;
		int month;
		month=3;
		
		int i1=2;
		int i2=3;
		
		month=i1+i2;
		
		switch(i1+i2)
		{
			case 1 :
			case 2 :
			case 3:
			case 4 :
			case 5:
				System.out.println("HI");
			case 6 :
			case 7:
			case 8:
			case 9:
			case 10: 
			case 11:
			case 12:
			
		}
		
		}
}
masijade
Who says anything wouldn't? The only pieces you need to be concerned about are the items directly following the keyword "case". Those are what must be compile-time constants. Everything else is irrelevant in that context.

Edit: And you should include a "default" case, of course, and you do realise that with that switch 1-5 will all print "HI", right?
843789
I think that "Compile time constant" is a literal constant or final variable which must be initialised at once.

So following code is correct 
   final int val3 = 3;		
  switch (key) {
	case 1:
		break;
			
	case 2:
		break	

	case val3:
		break;	

	}
But there is compilation problem:
  final int val3;	
  val3 = 3;	
  switch (key) {
	case 1:
		break;
			
	case 2:
		break	

	case val3:
		break;	

	}
And as we can see "...only final is not enough".
782681

punter wrote:
...what is a compile time constant ?...

it is defined in [JLS 15.28 Constant Expression|http://java.sun.com/docs/books/jls/third_edition/html/expressions.html#15.28|specification]

{noformat}
A compile-time constant expression is an expression denoting a value of primitive type
 or a String that does not complete abruptly and is composed using only the following:

    * Literals of primitive type and literals of type String (3.10.5)
    * Casts to primitive types and casts to type String
    * The unary operators +, -, ~, and ! (but not ++ or --)
    * ...

Compile-time constant expressions are used in case labels in switch statements (14.11)...
{noformat}

HTH

843789
So a compile time constant is the normal (final int CONSTANT_VARIABLE=4; ) constant that is declared ?

Thank you for your consideration.
782681
punter wrote:
So a compile time constant is the normal (final int CONSTANT_VARIABLE=4; ) constant that is declared ?

Thank you for your consideration.
1. I got an answer by writing a code and trying to compile it: 
final int CONSTANT_VARIABLE=4;
int var = 2;
switch (var) {
  case
    CONSTANT_VARIABLE:
      /* whatever */;
}
Above took me about 1 minute.

2. Then, I clicked the link to specification I gave you in my previous reply and studied it - in order to be 200% certain that answer gotten at step #1 is correct.
Above took me about 1 minute.

3. Then, I drafted a reply to you - the one you're reading right now.
Above took me about 2 minutes.

4. Then, I spent about 10 minutes doing nothing special, just killing my time prior to sending the reply. I did this in order to make a point for you.
Above took me... well about 10 minutes.

---------------------

Question - what point I was trying to make for you at step #4?
800308
gnat wrote:
Question - what point I was trying to make for you at step #4?
That some people have far too much time on there hands?
843789
Question - what point I was trying to make for you at step #4?
Point noted,thanks for your time and efforts. :)
800308
punter wrote:
Question - what point I was trying to make for you at step #4?
Point noted,thanks for your time and efforts. :)
Yessum... maybe 90% of "noob" questions can be answered with a simple experiment, taking less then 5 minutes to compile, run, and interpret.

I have a "template" java source code file called KRC.java (my initials) open in my text editor all the time... I just clear-out whatever garbage was in there previously and then chuck together an SSCCE to test any doubt I'm having about syntax, or how a library methods works, or whatever.

It's as effective as reading the API document, and accasionally many times more-so.

I wanted to know if the new(ish) for-each loop handled null collections gracefully... but I wouldn't even know if "language constructs" are documented in a searchable form analogous to the API doco.... so I tested it out myself.
package forums;

import java.util.List;
import java.util.ArrayList;

public class KRC
{
  public static void main(String[] args) {
    try {
      //List<String> list = new ArrayList<String>();
      List<String> list = null;
      for ( String item : list ) {
        System.out.println(item);
      }
    } catch (Exception e) {
      e.printStackTrace();
    }
  }
}
... and FYI... it causes an NPE at runtime. Ouch!

Cheers. Keith.
782681
corlettk wrote:
I have a "template" java source code file called KRC.java (my initials)...
mine "template" file for stuff like that is called JApp.java (J-ava App-lication) ;-)
It's as effective as reading the API document, and accasionally many times more-so.
so true
843789
corlettk wrote:
I have a "template" java source code file called KRC.java (my initials) open in my text editor all the time... I just clear-out whatever garbage was in there previously and then chuck together an SSCCE to test any doubt I'm having about syntax, or how a library methods works, or whatever.
I usually just bolt another unit test onto whatever tests I happen to have open in my IDE at any given point
Puce

gnat wrote:

punter wrote:
...what is a compile time constant ?...

it is defined in [JLS 15.28 Constant Expression|http://java.sun.com/docs/books/jls/third_edition/html/expressions.html#15.28|specification]

{noformat}
A compile-time constant expression is an expression denoting a value of primitive type
or a String that does not complete abruptly and is composed using only the following:

* Literals of primitive type and literals of type String (3.10.5)
* Casts to primitive types and casts to type String
* The unary operators +, -, ~, and ! (but not ++ or --)
* ...

Compile-time constant expressions are used in case labels in switch statements (14.11)...
{noformat}

HTH

Hmm, giving a quick check, I couldn't find the part that refers to enum constants...

Puce
Ah, an enum constant seems not to be a constant expression:
http://java.sun.com/docs/books/jls/third_edition/html/statements.html#14.11
843789
Puce wrote:
Ah, an enum constant seems not to be a constant expression:
Probably because you can use the unqualified enum name, even if you are not using any static imports.

That isn't possible in other places as well. A qualified enum name ("MyEnum.VALUE") should be a compile time constant expression.
Puce
JoachimSauer wrote:
A qualified enum name ("MyEnum.VALUE") should be a compile time constant expression.
I only scanned the constant expression part in the JLS, but I only see primitive and Strings mentioned, not enums. Can you point me to the part that fits to enums? Thanks.
782681

Puce wrote:

JoachimSauer wrote:
A qualified enum name ("MyEnum.VALUE") should be a compile time constant expression.

I only scanned the constant expression part in the JLS, but I only see primitive and Strings mentioned, not enums. Can you point me to the part that fits to enums? Thanks.

as far as I understand enums don't qualify as compile-time constants but nevertheless are allowed in switch case labels.

Per [JLS 14.11 The switch Statement|http://java.sun.com/docs/books/jls/third_edition/html/statements.html#14.11|spec]:

{noformat}
SwitchLabel:
        case ConstantExpression :
        case EnumConstantName :
        default :
{noformat}
Puce
Yes, that is what I understood, too.
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