Arraylist uses Array?

Question

Does an ArrayList internally use an Array? Is this an empty array if we use the default constructor (new ArrayList())? Thanks.

Answer 1

Yes.. ArrayList uses an array in itself.

It holds a Object array (in java. c# should be the same too) to give you a homogenity. Although arraylist seem to be a very dynamic memory allocating class, its internal operations are full of arrays.

The time when you create an object by calling constructor, internally it invokes an array of say a limited size, may be 10 (infact exactly 10 in java). Then as and when you add objects to the arraylist it will have to increase the internal array as well. So the internal array's size has to be increased. So, a new array is created with double the size and the old values are copied onto this new array. Note the capacity of the array is increased so more objects can be added.

Answer 2

Yes, an ArrayList uses an array to store the items.

If you create an ArrayList without specifying a capacity, the default starting capacity is used. What the default starting capacity is may depend on the version of the framework. For framework 2 it seems to be zero. In framework 1 i think that it was 16.

Answer 3

Yes it does. One of the easiest ways to verify this is to look at the source. You can either get your hands on the reference source, or you can simply use .NET Reflector to decompile the .NET DLLs.

Here's the relevant part of ArrayList, from Reflector:

public class ArrayList : IList, ICollection, IEnumerable, ICloneable
{
    static ArrayList()
    {
        emptyArray = new object[0];
    }

    public ArrayList()
    {
        this._items = emptyArray;
    }

    private object[] _items;

    private static readonly object[] emptyArray;

    // etc...
}

You shouldn't rely on this always being the case. It is an implementation detail and could change in future versions of .NET (although it probably won't). Also for new code you should consider using List<T> instead of ArrayList.