Java - getting loop iterator to a function inside loop?

Question

Kind of a silly problem I'm facing here... Basically I'm in a For-loop and within this loop I'm always calling a function to make a button. But in this function I want to pass the loop iterator as it's changing to differentiate the buttons. But then it tells me I need to make the loop iterator "final" which means it doesn't change!

Well my problem will make more sense with some skeleton code:

for(int i = 0; i < appList.size(); i++) { 

                //some other stuff including creating the below button 'btn'

                btn.setOnClickListener(new View.OnClickListener() {
                    public void onClick(View v) {
                        //set some attribute based on i
                        //but when I do this it only sets based on the greatest value of i (!)
                    }
                });
}

Meh kinda silly problem I know... I'm new to Java though!

Cheers.

The comment saying you will set the attribute based on i using the greatest value of i seems inaccurate. You shouldn't be able to compile code containing your snippet due to the lack of keyword final indicated in the answers. You may have wrapped your integer in a final single element array, though, and you'd get the behavior you describe. — Atreys
– Atreys, Commented Jun 14, 2011 at 15:37

Pablo Fernandez · Accepted Answer · 2011-06-14 14:57:04Z

4

for(int i = 0; i < appList.size(); i++) { 

    final int _i = i;

    btn.setOnClickListener(new View.OnClickListener() {
        public void onClick(View v) {
            // use _i instead of i
        }
    });
}

edited Jun 14, 2011 at 14:57

answered Jun 14, 2011 at 14:54

Pablo Fernandez

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1 Comment

Lukas Eder Over a year ago

_i must be declared final

Community · Accepted Answer · 2017-05-23 12:26:59Z

Consider this bit of silly example code:

int i = 0;
obj.invokeThunkAfter10Seconds(new ThunkObject() {
    public void thunk() {
        System.out.println(i);
    }
}
i = 1;

The ThunkObject holds a reference to i, but i can change, which makes the output unpredictable and depends on when i is changed and when thunk is invoked.

Because Java doesn't have proper closures (by design, see Cannot refer to a non-final variable inside an inner class defined in a different method), you are explicitly not allowed to capture non-final (i.e. changable) variables like this.

Instead you must declare the variable final, meaning it's initialized and then is not changable. This means you can never change i after it has been captured by the inner class.

final int i = 0;
obj.invokeThunkAfter10Seconds(new ThunkObject() {
    public void thunk() {
        System.out.println(i);
    }
}
// Can't change i here. It will forever now be 0.

In your example you can't just set i to be final, because you do want to change the value on each loop iteration. Instead you can create a copy of the value in a final variable:

for(int i = 0; i < appList.size(); i++) { 
    //some other stuff including creating the below button 'btn'

    final int capturedI = i;
    btn.setOnClickListener(new View.OnClickListener() {
        public void onClick(View v) {
            //set some attribute based on capturedI
        }
    });
}

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