Switch- Case doubt in case expressions.

[Ramesh Ponnada]

public class Sample {
static final int a=1;
static final int c;

static{
c=10;
}
public static void main(String... args) 
{
Sample s = new Sample();
int x=10;
switch(x){
case Sample.a: System.out.println("a");
case Sample.c: System.out.println("a"); // this is giving a case expressions must be compile time expressions. 
}
}
}

  
public class Sample {
final int a=1;
final int c;

{
c=10;
}
public static void main(String... args) 
{
Sample s = new Sample();
int x=10;
switch(x){
case s.a: System.out.println("a");  // now both case expressions are giving compile errors.
case s.c: System.out.println("a");
}
}
}

Question:

Both variables a and b are compile time constants as they are declared as final, even then why Compiler is complaining this way.
[ December 19, 2007: Message edited by: Ramesh Ponnada ]

[Keith Lynn]

Originally posted by Ramesh Ponnada:

public class Sample {
static final int a=1;
static final int c;

static{
c=10;
}
public static void main(String... args) 
{
Sample s = new Sample();
int x=10;
switch(x){
case Sample.a: System.out.println("a");
case Sample.c: System.out.println("a"); // this is giving a case expressions must be compile time expressions. 
}
}
}

  
public class Sample {
final int a=1;
final int c;

{
c=10;
}
public static void main(String... args) 
{
Sample s = new Sample();
int x=10;
switch(x){
case s.a: System.out.println("a");  // now both case expressions are giving compile errors.
case s.c: System.out.println("a");
}
}
}

Question:

Both variables a and b are compile time constants as they are declared as final, even then why Compiler is complaining this way.

[ December 19, 2007: Message edited by: Ramesh Ponnada ]

They are not compile-time constants.

In the first case, c is not assigned a value until the class definition is loaded into memory the first time.

In the second case, both a and c are instance variables so each instance of the class gets their own copy.

[Sergey Petunin]

Declaring a variable as final doesn't make it a compile-time constant.

According to the Java Language Specification, compile-time constant has to be initialized with the compile-time constant expression, which means that it has to have an initializer right where it's declared.

That's why in the first case "a" is a compile-time constant, and "c" is not.

Also, according to JLS, a compile-time constant may be composed of a simple name that refers to a constant variable or a qualified name of the form TypeName.Identifier that refers to a constant variable.

So in the second case you're using s.a and s.c, where s is not a type name, it's an instance name, so those are not compile time expressions.

[Cory Max]

I agree with Serge. If you substituted s. with Sample and set your var's when they were declared, the code would definitely compile.

    
public class Sample {
static final int a=1;
static final int c=10;
public static void main(String... args) 
{
            Sample s = new Sample();
            int x=10;
            switch(x){
                case Sample.a: System.out.println("a");  // now both case expressions are giving compile errors.
                case Sample.c: System.out.println("a");
            }
}
}

[Ramesh Ponnada]

Thanks guys,

Explanation really helped in understanding what are compile time constants.