Question

initialization of final static variable in a constructor

Asked by: perfect_tranquility

Here is the code: What is the issue with the "target line" ? Why Java does not allow the initialization of  a final static variable in a constructor?

Thanks,
R

public class class1 {
      private final int j ;
      private static int k;
      private final static int i;

class1()
{
//i=444; /* target line */
j=1;
k=22;
}

static{
      i=1;
      k=1;
       }

public static void main(String[] args) {new class1();
}
}

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Asked On
2007-10-16 at 15:06:36ID22897597
Tags

java

,

static

,

final

,

variable

Topics

Java Standard Edition

,

Java Programming Language

,

New to Java Programming

Participating Experts
5
Points
500
Comments
11

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Answers

 

by: objectsPosted on 2007-10-16 at 15:12:31ID: 20089496

because the declaration of it has already implicitly initilized it to 0
and u cannot change value of final vars

 

by: objectsPosted on 2007-10-16 at 15:13:50ID: 20089503

instead use:
      private final int j = 1;

 

by: UrosVidojevicPosted on 2007-10-16 at 15:14:17ID: 20089507

static final variable is a variable with constant value which belongs to class (not to objects).

It must be initialized before calling any constructor.
If it was intialized in every constructor it wouldn't be final (constant).
So you have to initialize it in the place of it's definition.

 

by: objectsPosted on 2007-10-16 at 15:14:47ID: 20089509

sorry, wrong line:

      private final static int i = 1;

 

by: TheMajesticPosted on 2007-10-16 at 20:09:40ID: 20090565

The thing is  

  The Static variables can be accessed without creating the object . i.e., 4They r intialized before creating the objects. if u r trying to intializing a variable of static inside the constructor its not possible.
rather u can use final alone.
     

 

by: SephsterPosted on 2007-10-17 at 16:00:31ID: 20097695

The very nature of the key word final means that once the variable is created, that is its final state. It cannot be changed after this. As you did not set a value to the i variable when creating it, it was automatically assigned the value of 0. However, in the constructor you have then tried to change it to the value of 1 which is not allowed. In order to rectify this simply remove i = 444; from the constructor and in the class variable section make a slight modification so it reads as below:

public class class1 {
      private final int j ;
      private static int k;
      private final static int i = 144;

class1()

 

by: malfunction84Posted on 2007-10-21 at 21:16:17ID: 20120781

Wow, there are a lot of misleading, contradicting answers here.  This only brings to light that "static" and "final" are really confusing, especially when they interact.

You DO NOT have to assign a value to a final variable when you declare it.  The "final" keyword just means that you can (and must) assign its value exactly once.  Where you are allowed to assign the value depends on if it is static or not (as well as its access modifier).

private final int j;
The value MUST be assigned ONCE, either here in the declaration or in each constructor.  It is NOT automatically assigned a value of 0 (zero).  Also, since it is not static, there is a distinct value for each instance object, so it makes sense that it must be assigned in the constructor.

private static int k;
The value may be assigned from anywhere in this class.  Since it is not final, the value may be assigned any number of times, but since it is static, this value is stored at the class-level and universally available to all instances.  If left unassigned, it is assumed to be 0 (zero).

private static final int i;
The value MUST be assigned ONCE, either here in the declaration or in a "static" class-level block.  It is NOT automatically assigned a value of 0 (zero).

So, to review:
"final": per-instance value, must be assigned once per instance.
"final" + "static": per-class value, must be assigned once per class.
"static": per-class value, can be assigned any number of times per class.
[neither modifier]: per-instance value, can be assigned any number of times per instance.

Before Java can create an instance of a class, it must first load and initialize that class.  One step of class initialization is assigning values to all its static fields.  If they're static AND final, those values can't change once they're assigned.

So, to answer your question, the reason you can't assign a value to a static final field in a constructor is that by the time you call the constructor, it should already have a value assigned which cannot be changed.

Hope this helps!

 

by: perfect_tranquilityPosted on 2007-10-26 at 16:17:50ID: 20159730

i started looking at the replies top down. I realized with minimal effort that there WAS NO DEFAULT value assigned to any final variable. either initialize it in a static block or just initialize it at the point of declaration.


it is surprising those whose answers run contrary to how java fundamentally works have not come back to comment and/or  update their comments.
 malfunction84 your id may say so but your performance has been fantastic. you have put in the effort and won!

R


 

by: perfect_tranquilityPosted on 2007-10-26 at 16:19:26ID: 20159734

also it's not always I award all the points to one answer. this case is a pointer to the fact that the selected answer is not only correct  but thorough!

R

 

by: perfect_tranquilityPosted on 2007-10-27 at 10:58:18ID: 20162294

Sorry Uros, your answer is theoretically correct too. I will mark you surely in your next response and make up.

R

 

by: malfunction84Posted on 2007-10-27 at 18:32:10ID: 20163389

Thanks, tranq.  I think you can contact the EE administrators if you want to distribute points to Uros for this question.  The Community forums are the place to go for that sort of thing.

20120131-EE-VQP-002

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