General anonymous class instantiation question

Question

I have noticed in the code in my system that someone instantiated an anonymous class as follows

 Class ExampleClass{

     MyObj obj; 

     methodA(new ClassA(){
         @override public void innerMethodA(){
            //code...
        }
    });
}

So far so good.
Now, in order to use obj that was declared before the method I usually define it as final.
I don't really understand why but i do because the compiler asks. In this code i see in innerMethodA() the usage of

 ExampleClass.this.obj()

without final.

My questions :
1. why do I have to put final when I use obj?
2. what is ExampleClass.this ? Notice that ExampleClass is the Class not an instance. then what is the "this"? if it has several instances?
3. What happens if I change the obj while the inner method runs (in my code inner method runs in a loop so I plan on changing it . will it explode?)

score 5 · Accepted Answer · answered Apr 28 '11 at 08:51

5

You have to use final when you capture the variable of a local variable... not an instance variable of the enclosing class.
ExampleClass.this is a reference to the instance of ExampleClass associated with the instance of the subclass of ClassA. In your case, it will be the same as this within methodA.
It won't explode - it will just change the value of obj. Think of it as capturing the value of ExampleClass.this (so you can't change that) but you can change the data within the object referred to by ExampleClass.this.

answered Apr 28 '11 at 08:51

Jon Skeet

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1. So, should I use every time ExampleClass.this.obj() instead of final? 2. Is it possible to cause a lock when I change its value ? – Bick Apr 28 '11 at 08:55
@user450602: It depends on what you're trying to use. For a local variable, you *do* need to make the local variable final. As for locking - you should probably make that part of the enclosing class. – Jon Skeet Apr 28 '11 at 08:58
@user450602: No, I mean exactly what I said. Do you understand the difference between instance variables and local variables? As for the locking part: if changing a value requires a lock to be acquired, you should write a method to do that consistently, and call that method from your inner class. – Jon Skeet Apr 28 '11 at 09:02

score 1 · Answer 2 · answered Apr 28 '11 at 08:52

There are no "true" closures (functions that capture scope) in Java. See e.g.http://stackoverflow.com/questions/1299837/cannot-refer-to-a-non-final-variable-inside-an-inner-class-defined-in-a-different
You can use this form to reference ambiguous methods / variables in the scope of "ExampleClass".
You cannot change the reference. What you can do is use an indirection, e.g. a final reference to an object that can swap it's values. An example would be the class of Atomic value holders, e.g. AtomicReference.

score 0 · Answer 3 · answered Apr 28 '11 at 08:53

Because, the way you described it, in this case, they are not using ExampleClass.this.obj, they are calling the method ExampleClass.this.obj().
ExampleClass.this refers to the encapsulating instance of ExampleClass in which this ClassA instance is instantiated.
Not necessarily.

General anonymous class instantiation question

3 Answers3