Initial capacity of ArrayList

[bugger]

I have declared an ArrayList and want to know its initial capacity.

Code:

ArrayList a = new ArrayList();

How to do it. ArrayList does not provide capacity() method as provided by Vector class.

[revathi17]

See if you get the answer here..

HTML Code:

http://www.java-forums.org/new-java/1812-use-arraylist-constructor.html

[bugger]

Thanks for the link but it does not address my problem.
I want to know the default capacity of an ArrayList. This is important to me because when ArrayList grows, its an expensive operation.

[wsaryada]

The default of initial capacity of ArrayList is 10, you can see it in the ArrayList source code.

[bugger]

Thanks wsaryada.

I cannot find capacity() method for getting the capacity of the ArrayList as in Vector.

How can I confirm this? Please give reference.

[wsaryada]

You can look in the ArrayList.java source code itself. It defined in the default constructor as follow:

Code:

   /**
     * Constructs an empty list with an initial capacity of ten.
     */
    public ArrayList() {
	this(10);
    }

You can find the source code in your JDK folder, in my PC it is C:\Program Files\Java\jdk1.6.0_03\src.zip

[ravian]

Great.
Thanks a dozen. Can we change this?? I mean are we allowed to change this??

[wsaryada]

Of course you can change the initial capacity of ArrayList. When creating the array list just pass the initial capacity you want in the constructor.

Code:

List list = new ArrayList(20);

This constructor will set the initial capacity of your array list to 20.

[ravian]

Yes. But can I change the default capacitiy of the ArrayList by changing ArrayList.java?

[wsaryada]

The constructor is the only way to change it from what I know. Other JDK implementation could have a different initial capacity, such a GNU JDK have 16 as the initial capacity. But again, passing the initial capacity through constructor will be enough.

[mew]

Using constructor is the easy way to do it. Why would you like to change the code in ArrayList.java.