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It's unclear to me what you're trying to do. If get(arg) gets anything above -1, it's going to call ArrayList.add(i, new HashMap()), which will add a hashmap to the array list as position i, enough times to reach arg. Forgetting, perhaps, that positions are 0-based so you'll "skip" a spot. No hashmap is being populated with any value (I think; it's hard to read).
So far there's nothing Struts-specific about this question, and no OGNL is shown in the code you've posted.