IndexOutOfBoundsException

[Luciform]

I appreciate there are a lot of problems with this error, particularly amongst newbies, but this one really is confusing me.

public class StockManager
{
    private ArrayList<String> database = new ArrayList<String>(10000);

The above is intended to create a new ArrayList with 10,000 indices. In a method later on the following statement attempts to execute:

database.add(product.id, product.details);

product.id is an integer, and product.details is a string. If my understanding of an ArrayList's add method is correct, the two parameters determine the index of the ArrayList element that data is to be added to, and then the data to be added.

When the above statement is reached in the method call, I get the error "IndexOutOfBoundsException: Index: 3055, Size: 0 (in java.util.ArrayList)" (product.id is equal to 3055).

What I don't understand is how can index 3055 be out of bounds when the ArrayList database has indices 0 to 9999? (I have double-checked that this is the case using the Object Inspector in my IDE.) If anyone can offer even the most painfully obvious fixes, I'd be very grateful.

[curmudgeon]

Constructing an ArrayList with a 1000 int parameter doesn't add 1000 items to the ArrayList, but rather gives the ArrayList an "initial capacity" of 1000 items. that is, this capacity is the size of the array that is held within the ArrayList object and that holds the items of the ArrayList. But this array works behind the scenes, and so giving the arraylist 1000 really just optimizes it for holding 1000 or slightly less items, nothing more. When your ArrayList starts out, even if it has a capacity of 1000, its size is still 0.

As an aside, wouldn't you be better off creating an ArrayList<Product> rather than one of String? This way you can simply add Products to the ArrayList at whatever position with the add(Object o) method.

[Luciform]

If that's the case, is it possible to change the size of the ArrayList, as opposed to the capacity?

[curmudgeon]

If that's the case, is it possible to change the size of the ArrayList, as opposed to the capacity?

Yes, by adding items to the ArrayList, but this begs the question of why? Again, I suspect that you're better off creating an ArrayList<Product> and just adding Product objects in without care for position. But if you want to add items to a collection in a sparse manner with the key or index being an int, consider using a HashMap<Integer, String> or HashMap<Integer, Product>.

[Luciform]

Thanks for your help curmudgeon. I think I can find a suitable workaround from those options.

[curmudgeon]

Cool!

¡Que tenga mucha suerte!

[banhbaochay]

Hi, Luciform,
AFAIK, your constructor ArrayList and add method are correct. The problem maybe in your product.id, is it continous? You can check it by printing to output all of product.id.
NOTE: ArrayList only can add object with continuous index. For example: I have this ArrayList as follow:

ArrayList array = new ArrayList<String> (10);

Then I add a object with index 2 before index 1 --> error

array.add(0, "werwe");
array.add(2, "asdfad");
array.add(1, "weroiu");

Or if I add object 0 and 2 (not add object 1), the same error:

array.add(0, "qweqw");
array.add(2, "safd");

This code must be change to:

array.add(0, "sasda");
array.add(1, "werw");
array.add(2, "asdfsdf");
...

The index must be continues: 0 --> 9 (in my case, and in your case: 0 --> 9999).
In your case, you can use Map, it stores data by hashtable type: key --> value

Sorry for my bad English,
banhbaochay

[curmudgeon]

NOTE: ArrayList only can add object with continuous index.

I think the requirement is that the index be between 0 and ArrayList.size(). For instance this is perfectly legal and in many situations, logical:

ArrayList array = new ArrayList<String> (10);
array.add(0, "werwe");
array.add(0, "asdfad");
array.add(0, "weroiu");

Sorry for my bad English,
banhbaochay

Your English is fine. :)

[banhbaochay]

Hi curmudgeon,
You're right, it's my fault :)
Thank you very much. My experience's increasing with your information :)