arrayList.ensureCapacity()

[Rich Stepanski]

I have an arrayList, and I'm adding objects to it at various positions. I use the arrayList ensureCapacity() to make sure the arrayList can accept a large subscript - but it doesnt seem to work? heres the code...

  
        array= new ArrayList();
        int pos=hashCode(f);
        
        // increase capacity if necessary    
        array.ensureCapacity(pos);      
                
        //check if LinkedList at position exists
        if(array.get(pos)==null){
            //add new LinkedList to array
            array.add(pos,new LinkedList());
        }

I've also tried to add() - not just trying to get() that brings up the same error. ( IndexOutOfBounds ) I also tried not using the ensureCapacity, I was under the impression ArrayList automatically expanded as it was necessary to fit items. ( I know they arent LinkedLists but would operate similiarly) Thanks for any help.

[Maulin Vasavada]

well the issue is indexing starts from 0 not 1

try,
//check if LinkedList at position exists
if(array.get(pos-1)==null){
//add new LinkedList to array
array.add(pos-1,new LinkedList());
}

thanks
maulin

[Maulin Vasavada]

i take that back. it might not work. i'm writing test program.

[Rich Stepanski]

i tried to just ensureCapacity(pos+1) - that didnt work either.. thanks for trying , any other ideas?

[Maulin Vasavada]

Hi Rich,

I found the problem. In the JDK1.4.2_03 source of ArrayList I see,

if (index > size || index < 0)
throw new IndexOutOfBoundsException(
"Index: "+index+", Size: "+size);

in add(index,object) method.

Now, the problem is when we call ensureCapacity(), it doesn't update the size as that call doesn't "add" any element in the array. Size variable reflects how many elements are there not how many it can hold.

So even when you call ensureCapacity(pos) size doesn't change and add(pos,data) fails as index > size

I guess you might have to override ArrayList's add(index,object) method to fit your needs...thinking more over it..

Regards
Maulin

[Maulin Vasavada]

So in essense we can only call add(index,object) method when we really want to "insert" in the array. Here the given index is surrounded by some elements already in the array.

Also, in the API it says "throws IAE if index is out of range of 0,size" so I guess its clear.

Now to solve your problem, I think you are trying some sort of hash table impl yourself. Why not create Integer as key and use Hashtable/HashMap? that will work for you...

Regards,
Maulin

[Rich Stepanski]

Thanks for the help, I'll try something different.

[Tony Morris]

http://java.sun.com/docs/books/tutorial/collections/index.html