enum semantics

[Marc Wentink]

Hi guys, please give me a little help again.

I still do not seem to be comfortable with enum semantics. I thought the code below would not compile. What is it with this static member that seems to be not a enum member but the whole enum class?

  
enum Animals
{
DOG, CAT
}
 
public class Test 
{
static Animals s_a; // que pasa aqui?
 
public static void main(String[] args) {
 
// This if for me the familiar way enums are used
System.out.println(Animals.DOG + " " +  Animals.CAT);
 
// But what is this then? I thought this would not compile
System.out.println(s_a.DOG + " " +  s_a.CAT);
 
// This obviously does not work
// Animals l_a = new Animals(); enum types may not be instantiated
 
// This if for me also the familiar way enums are used
Animals l_a = Animals.DOG;
System.out.println(l_a);
 
}
} 
 
It's partly taken from chapter one question 8.
 

[Burkhard Hassel]

Howdy gauchos,

in the line "que pasa" you declare a static enum variable of type Animals.
But you don't assign a value.
You can refer to this variable with Test.s_a because it is static.
Little hint:
Try this line as first line in your main method:

System.out.println(Test.s_a); // or just ...println(s_a);

Yours,
Bu.

[Marc Wentink]

Actually this is confusing me even more:

enum Animals {DOG, CAT}
 
public class Test {
static Animals s_a; 
public static void main(String[] args) {
 
System.out.println(s_a);  
//prints null
 
System.out.println(s_a.DOG + " " +  s_a.CAT); 
//prints DOG, CAT, so I can dereference a null value?
}
} 

[Yeming Hu]

Originally posted by Marc Wentink:
Actually this is confusing me even more:

enum Animals {DOG, CAT}
 
public class Test {
static Animals s_a; 
public static void main(String[] args) {
 
System.out.println(s_a);  
//prints null
 
System.out.println(s_a.DOG + " " +  s_a.CAT); 
//prints DOG, CAT, so I can dereference a null value?
}
} 

DOG && CAT are constant so when they're accessed, compiler will use the class name (here enum name) to replace the the reference variable. Therefore an object instance is not needed. In your example, the compiler will replace s_a.DOG to Animals.DOG. It's similar to the static variable of a class.
[ August 08, 2007: Message edited by: Yeming Hu ]

[camilo lopes]

e if was not static variavel? it would launch an exception? NullPointerException?

[Burkhard Hassel]

Aw, bad luck!

Marc said that

Actually this is confusing me even more

OK, you know already that the static variable s_a of class Test holds null in the first place.

But what you can do in a different class (of class Test itself - doesn't matter) is assigning enums of type Animals to that variable.

In your last example you only printed out the enums themself, you did not change the contents of the variable.
When you say System.out.println(s_a.DOG);
the s_a refers to Animals, and with the dot operator you select DOG. But this does not change the content of variable s_a.

Try this example:

class Other {
public static void main(String [] args) {
System.out.println(Test.s_a);
System.out.println(Test.s_a.DOG);
System.out.println(Test.s_a);
System.out.println("rereference");
[B]Test.s_a = Animals.CAT;[/B]
System.out.println(Test.s_a);
System.out.println(Test.s_a.DOG);
System.out.println(Test.s_a);
}
}

it prints:
null
DOG
null
rereference
CAT
DOG
CAT

s_a is just a class variable (i.e. a static one) that can hold an element of Animal.
At the beginning it is null, as would be any "normal" object reference, when no object is assigned to it.
Then you refer the dog but not change the value of s_a.
Only in the bold line, you change the value.


Yours,
Bu.

[Burkhard Hassel]

Oi ga�chos (*),

Camilo asked

e if was not static variavel? it would launch an exception? NullPointerException?

Howdy, Camilo, welcome to the ranch!





It doesn't matter if it is static or not. But it will cause a NullPointerException only when you try to call a method (or field) of it:

class StaticVersion {
static Animals besta;
public static void main (String[] alguns_argumentos){
System.out.println(besta); // prints null
System.out.println(StaticVersion.besta); // same: null
System.out.println(besta.hashCode()); // NPE!

}
}
 
class NonStaticVersion {
Animals besta;
public static void main(String[] args){
NonStaticVersion nsv = new NonStaticVersion();
System.out.println(nsv.besta); // null again
System.out.println(nsv.besta.hashCode()); // NPE!
}
}
 

NPE = NullPointerException.


See it?



Yours,
Bu.


---

(*) = Howdy, cowboys!

[Marc Wentink]

I think I understand it now. This one looks cute also by they way:

   
enum Animals {DOG, CAT}
 
public class Test {
static Animals s_a; 
public static void main(String[] args) {
System.out.println(s_a);                          // null
System.out.println(s_a.DOG + " " +  s_a.CAT);     // DOG CAT
s_a = s_a.DOG;   
System.out.println(s_a);                          // DOG
}
}
 

[ August 08, 2007: Message edited by: Marc Wentink ]