Declaring the capacity of a List in Java

Question

I often use Lists in my Android applications. Right now I am creating a Twitter page which lists a maximum of 50 "tweets" of a user.

I have a List defined like this:

List<Tweet> tweets = new ArrayList<Tweet>(MAX_TWEETS);

Where Tweet is a custom object type holding twitter update information(text, date, username, etc) and MAX_TWEETS is a constant integer value(50).

Questions:

What is the benefit of setting the initial capacity of this List, if any?

Should I bother setting a capacity when i know my list will be this small? When should i/should i not be setting a capacity?

Answer 1

Setting the initial capacity can increase the performance when populating the List, and it can also reduce the memory footprint of the List if you never add more than that number of items to the list.

The memory footprint of a List that has grown, and might have a backing array that is larger than the number of items stored can be reduced by invoking trimToSize()

Answer 2

By default, in Java 6, the size of a List is 10. That is, the system creates ten memory slots in the underlying Array. If you try adding the 11th element, only the Array copy is created. Providing a size improves performance.