Why it says that “Cannot refer to a non-final vari

问题:

This question already has an answer here:

• Cannot refer to a non-final variable inside an inner class defined in a different method 20 answers

I have button click listener and in onCreate() method I have a local variable like

 onCreate() {

 super.onCreate();

 int i = 10;

 Button button = (Button)findViewById(R.id.button);

 button.setOnClickListener(new View.OnClickListener() {

        @Override
        public void onClick(View v) {
            i++;
        }   
    });

Why java asks for to make me final ?

回答1:

When the onCreate() method returns, your local variable will be cleaned up from the stack, so they won't exist anymore. But the anonymous class object new View.OnClickListener() references these variables. Of cause it's wrong behavior so java don't allow you to do this.

After it is final it becomes a constant. So it is storing in the heap and can be safely used in anonymous classes.

回答2:

Your anonymous inner class refers to its enclosing scope by taking copies of the local variables - if you want to change the value of an int in an anonymous inner class, you need to do some hackery:

final int[] arr = new int[1];
arr[0] = 10;
Button button = (Button)findViewById(R.id.button);

button.setOnClickListener(new View.OnClickListener() {
  arr[0]++;
}

回答3:

Because you're accessing it within an anonymous inner class. You can't do that. You can make it final and then read it from the anonymous inner class, but you then can't increment it.

Options:

• Make it an instance variable of the outer class instead
• Make it an instance variable of the anonymous inner class instead
• Use a wrapper - e.g. a single-element array, an AtomicInteger or something like that

I would probably favour the second option unless I needed to get at i from anywhere else. I regard the third option as a bit of a nasty hack, to be honest.

回答4:

Making the variable final is necessary because under the hood, anonymous inner classes like that are simply syntactic sugar that compile to a nested class outside the scope of the containing method.

This means that none of the variables declared inside of the method are accessible to the inner class, so the compiler pulls another trick - and copies the value in a hidden constructor for your class. To avoid programmer confusion where this copy is not updated to match changes to the variable in the method, it must be final to make sure there are no such changes.

Since your goal here is to have an incrementing integer, and only the reference must be final (the object itself need not be immutable) you could declare a final AtomicInteger i and then increment it as you wish from the callback.

回答5:

Since you need to increment the i variable you can't make it final. You could make it a class member instead.