ArrayList exception outside of exception

Question

I have the following code:

ArrayList<Integer> arr = new ArrayList<Integer>(10); arr.set(0,5); 

I get the index outside of the error, and I don't know why. I declared an ArrayList size 10. Why am I getting this error?

Answer 1

You declared an ArrayList , which has an initial capacity of 10 elements, but you have not added an element to this list, i.e. the list is empty. set will replace an existing element, but since there is no element in the list, an exception is thrown. You must add elements earlier using the add method.

Initial capacity means that the array that stores the list inside has a size of 10 at the beginning. As more items are added to the list, the size of this internal array may change.

Answer 2

The original array is specified as "10", the actual number of elements in the array is 0.

To add “5” to the first index, just do arr.add (5)

The value passed to the ArrayList constructor is the initial capacity of the array backup storage. When you add items to a point that exceeds this capacity, inside the ArrayList will allocate a new size storage array and copy the items to a new storage array.

Answer 3

looking at the JDK source code ArrayList.set(int, E) gives a hint: before you can call set(N-1, E) , you need to have at least N elements.

Add your items using the add() method.

Answer 4

From the documentation :

Creates an empty list with the specified initial capacity.

(my emphasis)

Answer 5

What you passed in the constructor is just the initial capacity of the array by which this list is supported. After construction, the list remains empty. In addition, you should consider using a generic list if you want to keep ie only integers.

Answer 6

Use arr.add(0.5) . set method will replace the existing element.

Answer 7

set (int index, element E) Replaces the element at the specified position in this list with the specified element. U should use add ()

Answer 8

If you look at javadoc , pay attention:

Replaces the item at the specified position in this list with the specified item.

You need an item before you can replace it. Try

 arr.add(5); 

just add an item.

Answer 9

In the constructor you specified the initial capacity. However, the list size is still 0 because you have not added any items yet.

From the documentation of ArrayList.set() :

IndexOutOfBoundsException - if the index is out of range (index <0 || index> = size ()).