Why it says that "Cannot refer to a non-final variable i inside an inner class defined in a different method"?

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 ?