Tuesday, February 03, 2009


In one of my previous blogs, I mentioned a map of downloads based on Google Maps API. To create such a map, first I collect IP addresses from where the downloads were initiated, then I use GeoLite City to translate the IP address to a geographic location on the Google map. GeoLite City is a free library with Java API, but of less precision than its commercial counterpart.

It was working fine for many month until recently it threw out the infamous java.lang.NullPointerException. NullPointerException might be the No.1 runtime exception. It is certainly for me.

A little investigation explains why NullPointerException happened. When I passed an IP address to the GeoLite City library, I naively assumed that GeoLite must be able to translate it into a geographic location, thus I did not check whether the Location object returned by GeoLite is null. So I was pulished. In fact, IP address allocation is dynamic, and an outdated GeoLite City library may not contain all the IP address information.

It was not rare I made such a naive mistake. The fundamental principle to avoid NullPointerException is that, when given an object reference, no matter it is from my own code or from outside my code, thinking defensively, reason the possibility that it may be null. To play safe, always check nullity.

An object reference could be given as a returned value of a method invocation, as method arguments when inside a method, or as a field member of a non-null object.

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