Question

I have a function that iterates over a heavy data set, receives a callback (Google’s Guava Function) and runs the callback on every item of the data set:

void processData(..., Function<Item, Void> callback) {
  ...
  for (Item item : data) {
    callback.apply(item);
  }
} 

Using this function, I would like to pass a callback that adds all items to a list or a map:

List<Item> itemList;
processData(..., new Function<Item, Void)() {
  @Override public void apply(Item item) {
    itemList.add(item);
  }
});

However, it appears I can’t do that, since itemList is not final, and cannot be final by definition.

Is there a solution to this case? Or perhaps this entire pattern is wrong?

Answer 1

You seem to have a incorrect understanding of the final keyword. This statement:

final List<Item> itemList = new ArrayList<Item> ();

only means that the itemList variable can’t be reassigned. It does not prevent you from modifying the object it points to and this call:

itemList.add(someItem);

is perfectly valid on a final List. What is not allowed would be to write:

itemList = someOtherItemList; //forbidden, itemList is final