Experimenting with Datanucleus JPA

I have been using Hibernate for a very long time, and I'm more or less satisfied with it. When I saw that Google App Engine uses the JPA/JDO implementation of Datanucleus for high level persistence, I decided to create a sample JPA project with it.

Incremental average calculation

To calculate the average of some numbers, we can use the following simple formula:

It works in most cases, but let's suppose that we would like to know the average of a fast growing set of numbers.

Adding many items to a GWT ListBox - a faster approach

Adding items to a ListBox using the ListBox.addItem() method may result in performance problems on some browsers, which has been reported as issue#49.

For example, adding 10000 items in a loop takes (on my computer):

Browser	Time
FireFox 3.5.6	205 ms
Google Chrome 3.0.195.38	58 ms
Internet Explorer 8.0.6001.18702	33375 ms

The performance of IE8 is not acceptable, it's just too slow.
Fortunatelly there is a much faster method if you need to add many items when the ListBox is created (or it is possible to recreate the ListBox when it has to be repopulated).