Add object to ArrayList at specified index

Question

I think it s a fairly simple question  but I can t figure out how to do this properly   I ve got an empty arraylist   ArrayList lt object gt  list   new ArrayList lt object gt       I ve got some objects I want to add and each object has to be at a certain position  It is necessary however that they can be added in each possible order  When I try this  it doesn t work and I get an IndexOutOfBoundsException   list add 1  object1  list add 3  object3  list add 2  object2    What I have tried is filling the ArrayList with null and then doing the above  It works  but I think it s a horrible solution  Is there another way to do this

User · Answer

This is a possible solution   list add list size    new Object

User · Answer

I think the solution from medopal is what you are looking for   But just another alternative solution is to use a HashMap and use the key  Integer  to store positions    This way you won t need to populate it with nulls etc initially  just stick the position and the object in the map as you go along  You can write a couple of lines at the end to convert it to a List if you need it that way

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If that s the case then why don t you consider using a regular Array  initialize the capacity and put objects at the index you want   Object   list   new Object 10    list 0    object1  list 2    object3  list 1    object2

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You should set instead of add to replace existing value at index   list add 1  object1  list add 2  object3  list set 2  object2    List will contain   object1 object2

User · Answer

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

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I draw your attention to the ArrayList add documentation  which says it throws    IndexOutOfBoundsException - if the index is out of range  index  lt  0    index  gt  size     Check the size   of your list before you call list add 1  object1

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How about this little while loop as a solution   private ArrayList lt Object gt  list   new ArrayList lt Object gt      private void addObject int i  Object object        while list size    lt  i            list add list size    null             list add i  object           addObject 1  object1  addObject 3  object3  addObject 2  object2

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If you are using the Android flavor of Java  might I suggest using a SparseArray  It s a more memory efficient mapping of integers to objects and easier to iterate over than a Map

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You can use Array of objects and convert it to ArrayList-  Object   array  new Object 10   array 0   1   array 3    3   array 2   2   array 7   7    List lt Object gt  list  Arrays asList array     ArrayList will be-  1  null  2  3  null  null  null  7  null  null

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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  lt  arrayList size    index          System out println arrayList get index               Output would be ArrayList contains    1 INSERTED ELEMENT 2 3

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You need to populate the empty indexes with nulls   while  arraylist size    lt  position         arraylist add null      arraylist add position  object

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You could also override ArrayList to insert nulls between your size and the element you want to add     import java util ArrayList    public class ArrayListAnySize lt E gt  extends ArrayList lt E gt        Override     public void add int index  E element           if index  gt   0  amp  amp  index  lt   size                 super add index  element               return                    int insertNulls   index - size            for int i   0  i  lt  insertNulls  i                 super add null                     super add element             Then you can add at any point in the ArrayList   For example  this main method   public static void main String   args       ArrayListAnySize lt String gt  a   new ArrayListAnySize lt  gt         a add  zero        a add  one        a add  two        a add 5  five        for int i   0  i  lt  a size    i             System out println i      a get i                 yields this result from the console   0  zero  1  one  2  two  3  null  4  null  5  five

User · Answer

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

User · Answer

Bit late but hopefully can still be useful to someone   2 steps to adding items to a specific position in an ArrayList   add null items to a specific index in an ArrayList Then set the positions as and when required       list   new ArrayList     Initialise the ArrayList for  Integer i   0  i  lt  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

[java] Add object to ArrayList at specified index

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