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?

Answer 1

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);
   }