Final instance variable initialization behaviour

[aditya chaudhari]

Hi Everyone,

I am preparing for OCAJP 7 exam . My question is about final variable initialization and use.

What I Know : Instance variable get default initialized to values if not initialised with any value
eg. int get initialized to 0 , String initialized to null.

Then Why final instance variable not get initialised with value 0 in following code

public class Test {
   static final int i;
  // static int i
       
  
    public static void main(String[] args) {
      
        System.out.println(i);
        
    }
}

/* output
Test.java:c:error: variable i might not have been initialized
*/

Please help me to understand this, where i am doing mistake.?

[Sergej Smoljanov]

final variable always have no default value and must be internalized :
final static - if final static variable not initialed after static initialization block - compilation error
final instance - if final instance variable not initialized after constructor - compilation error

It is a compile-time error if a blank final (§4.12.4) class variable is not definitely assigned (§16.8) by a static initializer (§8.7) of the class in which it is declared.
A blank final instance variable must be definitely assigned (§16.9) at the end of every constructor (§8.8) of the class in which it is declared; otherwise a compile-time error occurs.

jls 7

[aditya chaudhari]

Thank You very much Sergej
you cleared my doubtS regarding final keyword.

It'll help me a lot.


Thank You

[Roel De Nijs]

Sergej Smoljanov wrote:final variable always have no default value and must be internalized :
final static - if final static variable not initialed after static initialization block - compilation error
final instance - if final instance variable not initialized after constructor - compilation error

Almost correct!

We have 3 kinds/types of variables. Let's discuss each of them.

class variables

Class variables (also called static fields, static variables,...) get a default value if you don't initialize them. If you use/access (e.g. for printing) a class variable which is not initialized, your code will still compile.
If you mark a class variable final, you MUST initialize this variable. You'll have 2 ways to do so: assign a value instantly OR in a static initializer block. If you don't initialize a final class variable, your code will NOT compile.

instance variables

Instance variables (also called data members, member variables,...) get a default value if you don't initialize them. If you use/access (e.g. for printing) an instance variable which is not initialized, your code will still compile.
If you mark an instance variable final, you MUST initialize this variable. You'll have 3 ways to do so: assign a value instantly OR in a constructor OR in an initializer block. If you don't initialize a final instance variable, your code will NOT compile.

local variables

Local variables (also called method variables,...) do NOT get a default value. If you use/access (e.g. for printing) a local variable which is not initialized, your code will NOT compile. But if a local variable is declared and not initialized, your code will compile if you don't use/access (e.g. for printing) this local variable. That's a little tricky! These lines can be summarized into 1 simple rule: you MUST assign a value to a local variable before you use/access it; otherwise your code will NOT compile.
If you mark a local variable final the same rules apply as to the (regular) local variables (so that's easy ).

I have made a little code snippet where all the different possibilities are mentioned. So you can start experimenting to get a good understanding of variable initialization behavior. (note: CE stands for Compiler Error)

class Finals {
    // private static final int finalClassVar1; // CE
    private static final int finalClassVar2 = 10;
    private static final int finalClassVar3;
    private static int classVar3;
    static {
        System.out.println(classVar3);
        // initialize in static initialization block
        finalClassVar3 = 100;
    }

    // private final int finalInstanceVar1; // CE
    private final int finalInstanceVar2 = 20;
    private final int finalInstanceVar3;
    private final int finalInstanceVar4;
    private int instanceVar1;

    public Finals() {
        System.out.println(instanceVar1);
        // initialize in constructor
        finalInstanceVar3 = 200;
    }

    {
        // initialize in initialization block
        finalInstanceVar4 = 200;
    }

    public void method() {
        final int finalVar1;
        final int finalVar2;
        final int finalVar3; // unused
        int var1;
        int var2;
        int var3; // unused
        // System.out.println(var1); // CE
        // System.out.println(finalVar1); // CE
        var2 = 3;
        System.out.println(var2);
        finalVar2 = 30;
        System.out.println(finalVar2);
        var2 = 300;
        System.out.println(var2);
        // finalVar2 = 15; // CE
    }
}

Hope it helps!

[aditya chaudhari]

Hi Roel ,

Thank You very much.

Your post above is perfect NOTES on final for my OCAJP 7 Exam.

I play with your code as well , Your code halps me to understand final variable behaviour better.

[Roel De Nijs]

aditya chaudhari wrote:Your post above is perfect NOTES on final for my OCAJP 7 Exam.

Don't forget my royalties

[aditya chaudhari]

Roel De Nijs wrote:
Don't forget my royalties

Yes Roel . All your post and other senior members posts in this forum helps me to prepare well.

I have marked this post as resolved.