Question

How better to test and create a new list, if it does not exist?

I only found this solution:

private List<List<Action>> actionList = Lists.newArrayList();

...

f(int index){
    Optional<List<Action>> optionalActionList = Optional.fromNullable(actionList.get(index));
    if (!optionalActionList.isPresent()) {
        actionList.add(new ArrayList<Action>());
    }
    actionList.get(index).add(index, new Action());
}

Maybe I should change the structure of storage? (The order of addition. Ability to add both the beginning and the end of the list. Are important in both lists).
Can use Google Guava if need.

Answer 1

It’s not clear what the Optional is really buying you here. I see the value of Optional as communicating between methods, or in storage.

Additionally, I don’t think the code is doing what you think it is – you’re asking for the list at a particular index, but then if the value at that index is null, you’re adding a new list at the end. It sounds like you want:

f(int index){
    List<Action> list = actionList.get(index);
    if (list == null) {
        list = new ArrayList<Action>();
        actionList.set(index, list);
    }
    // Note: no need to use index at all here... potentially
    list.add(new Action());
}

Note that I’m not using add(index, new Action()) at the end – at it was very unclear why you’d want to do so. You’ve already got to the right list using index – why would you use index again?

The above assumes that the “outer” list is already the right size, but filled with nulls for “absent” lists. If that’s not the case, you’ll need to detect when index is beyond the bounds of the current outer list.

Basically, if the above doesn’t help, please give more information as your question is currently unclear.