Adding items to empty List at specific locations in java

Is there any way I can make the below code work without commenting the 3rd line.

    List<Integer> list = new ArrayList<Integer>();
    list.add(0,0);
    //list.add(1,nul开发者_运维知识库l);
    list.add(2,2);

I want to add items to list at specific locations. But if I don't change the index to Nth position I am not being able to add at Nth as told in this answer.

I can't use a map because I don't want to miss a value when the keys are same. Also adding null values to a list for large lists will be an overhead. When there is a collision I want the item to take the next position(nearest to where it should have been).

Is there any List implementation that shifts index before it tries to add the item?

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Use something like a MultiMap if your only concern is not "missing a value" if the keys are the same.

I'm not sure how doing a shift/insert helps if I understand your problem statement--if the "key" is the index, inserting will lose the same information.

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You can use Vector and call setSize to prepopulate with null elements.

However, your comment about the overhead of the nulls speaks to an associative container as the right solution.

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This still smells like you should be using a Map. Why not use a Map<Integer, List<Integer>>?

something like,

   private Map<Integer, List<Integer>> myMap = new HashMap<Integer, List<Integer>>();

   public void addItem(int key, int value) {
      List<Integer> list = myMap.get(key);
      if (list == null) {
         list = new ArrayList<Integer>();
         myMap.put(key, list);
      }
      list.add(value);
   }

   public List<Integer> getItems(int key) {
      return myMap.get(key);
   }

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Well, There are a couple of ways I would think to do this, if you are not adding items too frequently, then it might be a good idea to simply do a check to see if there is an item at that location before adding it.

if(list.get(X) == null)
{
 list.add(X,Y);
}

Otherwise if you are going to be doing this too often...then I would recommend creating your own custom List class, and extending ArrayList or whatever you are using, and simply override the add method, to deal with collisions.