Final variables

[Abhi vijay]

It is said that local method inner classes cannot have access to the non-final variables of the method in which they are defined.

  
class test
{
	
   public static void main( String args[] )
   {
	   final int w;
	   w=2;
      class ma
      {
         final int x;
         ma ()
         {
            x = 10;
            System.out.println( this.x);
              
                System.out.println( w);
         }
      }
      new ma();

   }
}
  

Then why does this work?

[Rajshekhar Paul]

because w is final int.

[Punit Singh]

What happened Abhi, are your eyes loosing power ?

[Abhi vijay]

No, Punit .
Can you tell me why local method inner classes cannot access the non-final variables of the method??

[Ankit Garg]

Abhi when you create a method local inner class, it gets a copy of all the final constants of a method and the final parameters passed to the method. it doesn't get a copy of non-final variables. This is why is is mandatory to initialize all the final constants of a method before declaring a method local inner class or an anonymous inner class...

[Punit Singh]

Abhi vijay wrote:No, Punit .
Can you tell me why local method inner classes cannot access the non-final variables of the method??

1) As local variables lives on stack, and your innerclass object will live on heap.
2) You can pass your innerclass object's reference outside your method, and it will be still accessible after your method ends.
3) local variables will garbage collected after your method ends.

[Abhi vijay]

what about final local variables??

[Punit Singh]

Abhi vijay wrote:what about final local variables??

Ankit said na already, that final is not ganna change in its life, so innerclass object takes it copy for ever.

[Abhi vijay]

I meant to ask, what about final local variables which are not compile time constants? Is it the same?? As w is not a compile time constant.

[James Tharakan]

what about final local variables??

If you decompile a program you would see that the final variables are replaced in all the places where it is refered.

[Punit Singh]

Abhi vijay wrote:I meant to ask, what about final local variables which are not compile time constants? Is it the same?? As w is not a compile time constant.

That is a completely different story, I have seen a post here where sheriffs are talking about Permanent storage where final local runtime variables is stored, you can say final local variable are not stored in stack, so it can be used. There is storage funda behind this concept.

[Ankit Garg]

Abhi vijay wrote:I meant to ask, what about final local variables which are not compile time constants? Is it the same?? As w is not a compile time constant.

Upto what I understood, you want to ask is whether the inner class get's a copy of the final fields in a method which are compile time constants. Well yes the inner class get's a copy of these fields. And as I said before since the inner class get's a copy of them, so you must instantiate them before you declare the local class (only if the local class uses the field)...

[Edit: added a minute detail ]

[Abhi vijay]

Ok.Thanks. Ankit and Punit.

[Punit Singh]

I do not much about it, but Ankit has done lots of R&D over this topic. You can find something in Ankit's blog.

I just made your example:

  public class LocalInnerClassAndFinalVar  
 {  
       
    public static void main( String args[] )  
    {  
        final int w;  
        w=2;  
       class ma  
       {  
           int x;  
          ma ()  
          {  
             x = 10;  
             System.out.println( this.x);  
                 
                 System.out.println( w);  
          }  
       }  
       new ma();  
   
    }  
 }   

I decompiled two class file generated by this class.
And got main file here:LocalInnerClassAndFinalVar.class

   import java.io.PrintStream;

public class LocalInnerClassAndFinalVar
{

    public LocalInnerClassAndFinalVar()
    {
    }

    public static void main(String args[])
    {
        final byte w = 2;//see this line changed
        class _cls1ma //see class name changed
        {

            int x;
            final int val$w;//see new final variable declared inside

            _cls1ma()
            {
                w = i;//see this is what unexpected thing.
                super();
                x = 10;
                System.out.println(x);
                System.out.println(w);
            }
        }

        new _cls1ma();
    }
}
 

and inner class file is here:LocalInnerClassAndFinalVar$1ma.class

  import java.io.PrintStream;

// Referenced classes of LocalInnerClassAndFinalVar

class val.w//see its name is matching with final variable name in above file
{

    int x;
    final int val$w;

    ()
    {
        val$w = i;
        super();
        x = 10;
        System.out.println(x);
        System.out.println(val$w);
    }
}
  

Ankit could say more about this.

[Punit Singh]

And this one when I put final int w as compile time constant, above was for run-time constant.

Actual code:

 public class LocalInnerClassAndFinalVar  
 {  
       
    public static void main( String args[] )  
    {  
        final int w=2;  
       
        class ma  
       {  
           int x;  
          ma ()  
          {  
             x = 10;  
             System.out.println( this.x);  
                 
                 System.out.println( w);  
          }  
       }  
       new ma();  
   
    }  
 }    

Decompiled code:
LocalInnerClassAndFinalVar.class

  import java.io.PrintStream;

public class LocalInnerClassAndFinalVar
{

    public LocalInnerClassAndFinalVar()
    {
    }

    public static void main(String args[])
    {
        class _cls1ma
        {

            int x;

            _cls1ma()
            {
                x = 10;
                System.out.println(x);
                System.out.println(2);//see w is replaced by 2 here
            }
        }

        new _cls1ma();
    }
}  

and decompiled inner class:
LocalInnerClassAndFinalVar$1ma.class

  import java.io.PrintStream;

// Referenced classes
//            LocalInnerClassAndFinalVar

class x
{

    int x;

    ()
    {
        x = 10;
        System.out.println(x);
        System.out.println(2);//see w is replaced by 2
    }
}
  

[Ruben Soto]

That's very interesting. Maybe I need to start reading Ankit's blog.
I don't really understand the line
val$w = i;

[Ankit Garg]

Ruben Soto wrote:I don't really understand the line
val$w = i;

It's just a copy of the final field of the method that the local class keeps as a copy for later referencing...