Why method local inner class can't access non final local variable of method?

[V. Potluri]

Hi,

  
        class Outer {
             int x = 11;
             public void m1() {
                final int a = 10;
                int b = 20;
                class Inner {
 
                   System.out.println("x = "+x); 
                   System.out.println("a = "+a);
                   System.out.println("b = "+b);//Causes compile time error
 
                }
              }
         }
 

Can any one please explain, why can't we access method local variable b inside method of Inner class

Thanks in advance.

Have a wonderful day ahead!

Regards,
Gopal

[Henry Wong]

BTW, does the code compile? Even if you remove the line that causes the compile time error, I don't see how your code compiles.

Henry
[ October 22, 2008: Message edited by: Henry Wong ]

[Jean-Luc Thirion]

For method-local inner classes, only "compile time constants" are available. a is final and initialized at declaration, so the compiler knows it at compile time.

b is a normal, stack variable, and is not a "compile time constant". Think at it, outside of the method b will disapear (out of scope). An instance of the internal class could be linked to another object whose lifetime is much much longer,for instance:

code 1:
ArrayList list = new ArrayList<Integer>();
x.m1(list ); //imagine a call to m1, from another class far away
code 2:
public void m1(List<Integer> a) { 
  Integer b =  20;
  class Inner() {
      Integer x;
      Inner() {this.x = b;}
      void registerMe(List<Integer> a) {
         a.add(this);
      }
  } 
  
  Inner i = new Inner();  //b is still alive
  i.registerMe(a);  //i is linked to the arraylist a
  
  //Now the method will finish, b is out of scope and may be garbage collected
}
 

 

After call of m1(list). List will stay alive and be filled with an object of type Outer.Inner, which will have a reference to b who may be garbage collected

[Henry Wong]

For method-local inner classes, only "compile time constants" are available. a is final and initialized at declaration, so the compiler knows it at compile time.

This is not true. It doesn't have to be a compile time constant. It just needs it to be a constant.

Basically, during instantiation of the inner class, when both the method and the class is in scope, the inner class will make a copy of the variables which are constants. This means, that the method can go out of scope, since the inner class is only using a copy of the variable.

Henry

[Tuna Töre]

int b

in your code will be disappear after your method execution completes, therefore you will be allowed to use only final and instances variables with method local inner classes



[ October 22, 2008: Message edited by: Anut Walidera ]

[Ankit Garg]

you can watch my blog jgenius.blogspot.com/ if it would clear your doubt...

[victor kamat]

Go to this site for the answer:
http://techtracer.com/2008/04/14/mystery-of-accessibility-in-local-inner-classes/