Add object to ArrayList at specified index

Answer 1

@Maethortje 

The problem here is java creates an empty list when you called new ArrayList and 

while trying to add an element at specified position you got IndexOutOfBound , so the list should have some elements at their position.

Please try following

/*
  Add an element to specified index of Java ArrayList Example
  This Java Example shows how to add an element at specified index of java
  ArrayList object using add method.
*/

import java.util.ArrayList;

public class AddElementToSpecifiedIndexArrayListExample {

  public static void main(String[] args) {
    //create an ArrayList object
    ArrayList arrayList = new ArrayList();

    //Add elements to Arraylist
    arrayList.add("1");
    arrayList.add("2");
    arrayList.add("3");

    /*
      To add an element at the specified index of ArrayList use
      void add(int index, Object obj) method.
      This method inserts the specified element at the specified index in the
      ArrayList.  
    */
    arrayList.add(1,"INSERTED ELEMENT");

    /*
      Please note that add method DOES NOT overwrites the element previously
      at the specified index in the list. It shifts the elements to right side
      and increasing the list size by 1.
    */

    System.out.println("ArrayList contains...");
    //display elements of ArrayList
    for(int index=0; index < arrayList.size(); index++)
      System.out.println(arrayList.get(index));

  }
}

/*
Output would be
ArrayList contains...
1
INSERTED ELEMENT
2
3

*/

Answer 2

You can do it like this:

list.add(1, object1)
list.add(2, object3)
list.add(2, object2)

After you add object2 to position 2, it will move object3 to position 3.

If you want object3 to be at position3 all the time I'd suggest you use a HashMap with position as key and object as a value.

Answer 3

I draw your attention to the ArrayList.add documentation, which says it throws IndexOutOfBoundsException - if the index is out of range (index < 0 || index > size())

Check the size() of your list before you call list.add(1, object1)

Answer 4

You need to populate the empty indexes with nulls.

while (arraylist.size() < position)
{
     arraylist.add(null);
}

arraylist.add(position, object);

Answer 5

Suppose you want to add an item at a position, then the list size must be more than the position.

add(2, item): this syntax means, move the old item at position 2 to next index and add the item at 2nd position.

If there is no item in 2nd position, then this will not work, It'll throw an exception.

That means if you want to add something in position 2,

your list size must be at least (2 + 1) =3, so the items are available at 0,1,2 Position.

in that way it is ensured that the position 2 is accessed safely and there would be no exception.

Answer 6

Bit late but hopefully can still be useful to someone.

2 steps to adding items to a specific position in an ArrayList

1. add null items to a specific index in an ArrayList

2. Then set the positions as and when required.

       list = new ArrayList();//Initialise the ArrayList
   for (Integer i = 0; i < mItems.size(); i++) {
       list.add(i, null); //"Add" all positions to null
   }
      // "Set" Items
       list.set(position, SomeObject);
   

This way you don't have redundant items in the ArrayList i.e. if you were to add items such as,

list = new ArrayList(mItems.size());    
list.add(position, SomeObject);

This would not overwrite existing items in the position merely, shifting existing ones to the right by one - so you have an ArrayList with twice as many indicies.