Adding items to empty List at specific locations in java

Question

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,null);
    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?

Use a TreeMap since that allows iteration of the keys in order. — John B
– John B, Commented Oct 11, 2011 at 16:48
What's the desired behaviour when you have a collision at a key? — Mark Peters
– Mark Peters, Commented Oct 11, 2011 at 16:50
@MartijnCourteaux: I'm not sure. His mention of "can't use a map because I don't want to miss a vlaue when the keys are the same", suggested that he wanted multiple values for one key, but I could be wrong. As some have mentioned, the original post is not very clear about this and the original poster appears to be in no hurry to clarify this. — Hovercraft Full Of Eels
– Hovercraft Full Of Eels, Commented Oct 11, 2011 at 16:59

Dave Newton · Accepted Answer · 2011-10-11 16:48:02Z

5

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.

answered Oct 11, 2011 at 16:48

Dave Newton

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1 Comment

John B Over a year ago

Specifically, a TreeMultimap provides in-order iteration of the keys.

Steve Townsend · Accepted Answer · 2011-10-11 16:50:41Z

3

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.

answered Oct 11, 2011 at 16:50

Steve Townsend

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1 Comment

ManuPK Over a year ago

I think we need setSize() method in List also.It can be useful.

Hovercraft Full Of Eels · Accepted Answer · 2011-10-11 16:55:58Z

2

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);
   }

edited Oct 11, 2011 at 16:55

answered Oct 11, 2011 at 16:47

Hovercraft Full Of Eels

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Comments

Suraj Kulkarni · Accepted Answer · 2011-10-11 21:15:54Z

2

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.

answered Oct 11, 2011 at 21:15

Suraj Kulkarni

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2 Comments

ManuPK Over a year ago

Yes. I wanted the check to happen automatically and increase the size if it is less. Looks like I need to think about a custom list implementation to do it.

Martin Mucha Over a year ago

this simply does not work at all, since java.util.ArrayList#add(int, E) does do range check: if (index > size || index < 0) So for example you cannot add element at first index into empty list.

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