Switch Case constants

[MukulJ Patil]

I am trying to compile following program but it's giving error- that "Switch case constants must be Compile time constants!" This can be removed by declaring & initialising the final variable on the same line. But i am doing declaration & initialisation on two different lines. I don't know the difference between these two. Please explain.
public static void main(String[] args)
{
final int a=0;
final int b;
b=10;
int x=0;
switch(x)
{
case a: System.out.println("ok1"); break;
case b: System.out.println("ok2"); break; // Error over here. }
}

[Pradeep Kadambar]

Hi Mukul,

Your code would compile fine if suppose you do something like

    
final int b = 10;

If b is not constant then it may change over the course of execution ! which is not intended. So you always need to have constant or expressions that can be evaluated at compile time.

But in your code

    
final int b = 1;
b = 10;          // This is error as b is final

[MukulJ Patil]

I know how to make it compile. But my question is what is the difference in the two types of declarations?

[Pradeep Kadambar]

See there is a big difference b/w

   
final int b;      // blank final - Just b'coz this is final doesn't mean value is known at compile time. For your switch case compiler expects compile time constant !!!
b = 10;

and a

    
final int b = 10; // Value inlined at compile time 

[vitthal wable]

Hii,
You are Using variable which are initialize at execution time ,
that is

 fianl int b;
                        b=10;   
                 

isintialize at run time,
As switch requires the variable which are initialized compile time
means declared and initialized on same line

[MukulJ Patil]

Guys,
Thanks.
But you all are talking about the things that i have already stated. I am interested in knowing why there is such difference in two forms of declaration? In both cases, i am initialising the variable. Then why it causes the compiler to treat the two line declaration as runtime initialisation?

[Tony Morris]

Because one is a constant and one is not.
The definition of a constant is given in JLS3 15.28. You might also find this interesting: When a final is not a constant
[ September 19, 2006: Message edited by: Tony Morris ]

[Tony Morris]

Sorry about the link screw up - fixed now.