Java - changing the value of a final variable from within a lambda

Question

In Java I have the following code

List myList = new ArrayList<>();
for (int i=0;i<9;i++) {
    myList.add(i);
}

Integer sum = 0;

m         


        

Answer 1

Try with:

final Integer[] sum = new Integer[1];
sum[0] = 0;

myList.forEach(i -> {
    sum[0] = sum[0] + i; // does not compile, sum needs to be final or effectively final
}); 

Since lambda is actually a syntactic sugar for initializing an anonymous class (and overriding a method).

It's the same as if you have written:

final Integer[] sum = new Integer[1];
sum[0] = 0;

myList.forEach(new Consumer() {
    public void accept(Integer element) {
        sum[0] = sum[0] + element;
    }
});

The variable that comes from outer scope and that you use within inner scope must be final (in this example sum). That is simply because Java does not support closures. Therefore, outer variable must be marked as final. Since Integer itself is immutable (if you declare it final, you cannot change it anymore), you have to use a wrapper object or an array (as I did).

You can find more useful info here:

• Why are only final variables accessible in anonymous class?
• Cannot refer to a non-final variable inside an inner class defined in a different method