Franz

How to write useful error messages

Each programmer has to deal with error mesages in some way. Either we have to check our data (and possibly raise an error/exception) or we are using libraries and have to deal with error messages or exceptions that are raised in these libraries. The purpose of this post is to point out how meaningfull error messages can save you time. Either your own time (by avoiding answering support questions) or your own debugging time.

So how can an error message save or waste time? Imagine the situation where you try to open a file and all you get is an error. What do you do next? I guess you’ll probably check if you’re trying to open the correct path. Maybe you did, then you possibly check if the permissions are okay, if you’re trying to open a directory or a file, etc until you found the problem that is causing the error. Actually this can be quite some wasted time – why? Becasue you’re doing all the stuff that the system already checked. The system decided that one of the condidtions failed and exited. So – shouldn’t the system just TELL the exact reason? And even better: shouldn’t it also just tell you a possible solution? Well – actually I do not blame “the system” but the guy who write the line that threw the error without any further information. Everytime I get an absolutely uninformative error I think “was it really so damn hard to add one more line of code that just mentions WHAT went wrong?!”.

Assume the situation where you try to open a file with the path being configured in an external config.properties file.
I think the quality of an error message can be categorized in one of the three categories:

Reporting: The system just reports that there was an error.
Example: IO Exception occured.
Informing: Same as 1, just with more information.
Example: IO Exception occured while opening file: <filename>, null=false, exists=true, is file=false, is directory=false, readable=false, writable=false
Supporting: same as 2 with additional information of how to solve the problem.
Example: IO Exception occured while opening file: <filename>, null=false, exists=true, is file=false, is directory=true, readable=false, writable=false; Was the pat set correctly in config.properties?

In the past weeks I have seen very useful error messages in Google’s Android (“… Have you declared this activity in your AndroidManifest.xml?”) or in twitter4j (Displaying a shortURL to an FAQ page explaingin a very common error).
Bad examples are plain NullPointerExceptions when 3 parameters were checked, one of them was null and the according is just followed by a throw new NullPointerException so that you even did not recognize from the message WHICH parameter was null.

Conclusion: Please write robust code and please provide helpful and supportive error messages to the folks using your code!

An introduction to the Logging framework (a.k.a. System.out.println is evil)

Motivation

One of the first lines a programmer will write in a new language is surely “Hello World”. In Java you can write to the console or to the error stream quite easy with a simple System.out.println("Hello World") or System.err.println("Hello Error"). Great! When the code grows, bugs creep into the code and make live a bit harder. At this point programmers should definately start a deep and loving relationship with the debugger that is delivered with the IDE instead of using System.out/err.println() as debug method. Nevertheless – there are at cases, where a Debugger cannot (or hardly) be applied:

The code runs in the IDE but not if startet directly. – What the hell’s going wrong?
Handling of exceptions. An exception indicates a state that should not have happened and therefore it might be considered to be logged.
The code is deployed to someone else and you cannot attach the debugger to his/her machine.

At either point, beginners tend to use System.out/err.println() to trace the execution path. While this might be okay if the onlyone that is using the code is the programmer alone, this can be very annoying if you are working in a team: If you forget to remove the debug messages, you’re polluting someone elses console output. Even worse: if the code is deployed to a client which reports an error, you cannot raise/lower debug levels or just enable/disable debugging. Do you really want to send a “debug version”? (No you wouldn’t.)

Continue reading An introduction to the Logging framework (a.k.a. System.out.println is evil)

Rooting and upgrading my HTC Tattoo to CyanogenMod 7

Yesterday I decided that the days of my HTC Tattoo running original HTC-Android-1.6 were numbered – finally. Actually the thought about updating was in my mind for QUITE some time. But with all the different how-to-update guides and “oh my god I broke my phone”-posts, I was really frustrated and unsure if I should really risk to “brick” my phone (a.k.a.: turn it totally unusable).

Well okay – so I crawled the web once more for an up-to-date update guide. Well – to make a (rather) long story short: I achieved my goal!

Continue reading Rooting and upgrading my HTC Tattoo to CyanogenMod 7

Parse Error: There is a problem parsing the package / Beim Parsen des Pakets ist ein Problem aufgetreten

Just finished my first (real) Android App and wanted to deploy it in my phone (not just in the emulator).
So I sent an email to myself with the .apk attached as several tutorials said that you can just open it from your mail to install the app.
On the phone, I opened my mail app K9 and opened the .apk. But instead of installing I unfortunately just got

Beim Parsen des Pakets ist ein Problem aufgetreten.
(in english) Parse Error: There is a problem parsing the package.

even though the minSdkVersion was set correctly.

The solution was rather simple: Do NOT open the .apk in K9 directly but save it to the sdcard and open it via a filemanager like Astro.

Knowing: A Generic Data Analysis Application

We got another demo accepted:

Knowing: A Generic Data Analysis Application

Thomas Bernecker, Franz Graf, Hans-Peter Kriegel, Nepomuk Seiler, Christoph Türmer, Dieter Dill
To appear at 15th International Conference on Extending Database Technology (2012)
March 27-30, 2012, Berlin, Germany

Abstract:

Extracting knowledge from data is, in most cases, not restricted to the analysis itself but accompanied by preparation and post-processing steps. Handling data coming directly from the source, e.g. a sensor, often requires preconditioning like parsing and removing irrelevant information before data mining algorithms can be applied to analyze the data. Stand-alone data mining frameworks in general do not provide such components since they require a specified input data format. Furthermore, they are often restricted to the available algorithms or a rapid integration of new algorithms for the purpose of quick testing is not possible. To address this shortcoming, we present the data analysis framework Knowing, which is easily extendible with additional algorithms by using an OSGi compliant architecture. In this demonstration, we apply the Knowing framework to a medical monitoring system recording physical activity. We use the data of 3D accelerometers to detect activities and perform data mining techniques and motion detection to classify and evaluate the quality and amount of physical activities. In the presented use case, patients and physicians can analyze the daily activity processes and perform long term data analysis by using an aggregated view of the results of the data mining process. Developers can integrate and evaluate newly developed algorithms and methods for data mining on the recorded database.

BibTex

@INPROCEEDINGS{BerGraKriSeietal12,
  AUTHOR     = {T. Bernecker and F. Graf and H.-P. Kriegel and N. Seiler and C. Tuermer and D. Dill},
  TITLE      = {Knowing: A Generic Data Analysis Application},
  BOOKTITLE  = {Proceedings of the 15th International Conference on Extending Database Technology (EDBT), Berlin, Germany},
  YEAR       = {2012}
}

More informations will be published at the official publication site at the LMU.

Research Idea: Evaluation of Traffic Lane Detection with OpenStreetMap GPS Data

I am soon leaving University and thus the time for pure research will soon be over. Unfortunately I still have some ideas for possible research. I’ve tried getting them out of my head as this has not yet worked out, I’ll try to write them down – maybe somewone finds them interesting enough for a Bachelor-/Masterthesis or something like that …

Introduction

OpenStreetMap creates and provides free geographic data such as street maps to anyone who wants them. The project was started because most maps you think of as free actually have legal or technical restrictions on their use, holding back people from using them in creative, productive, or unexpected ways. The OpenStreetMap approach is comparable to Wikipedia where everyone can contribute content. In openStreetMap, registered users can edit the map directly by using different editors or indirectly by providing ground truth data in terms of GPS tracks following pathes or roads. A recent study shows, that the difference between OpenStreetMap’s street network coverage for car navigation in Germany and a comparable proprietary dataset was only 9% in June 2011.

In 2010, Yihua Chen and John Krumm have published a paper at ACM GIS about “Probabilistic Modeling of Traffic Lanes from GPS Traces“. Chen and Krum apply Gaussian micture Models (GMM) on a data set of 55 shuttle vehicles driving between the Microsoft corporate buildings in the Seattle area. The vehicles were tracked for an average of 12.7 days resulting in about 20 million GPS points. By applying their algorithm to this data, they were able to infer lane structures from the given GPS tracks.

Adding and validating lane attributes completely manually is a rather tedious task for humans – especially in cases of data sets like OpenStreetMap. Therefore it should be evaluated if the proposed algorithm could be applied to OpenStreetMap data in order to infer and/or validate lane attributes on existing data in an automatic or semiautomatic way.

Continue reading Research Idea: Evaluation of Traffic Lane Detection with OpenStreetMap GPS Data

SRTM Plugin for OpenStreetMap

One of the features of our TrafficMining Project at the LMU was to use additional attributes in routing queries. Standart routing queries usually just use time and distance for calculating the fastest/shortest routes. In order to have an additional attribute we decided to use evelation data as this might be an issue if you also want to take fuel cost into account or if you’re planning a bike tour (instead of crossing a hill, you might want to consider a longer tour that avoids crossing the hill).

The problem just was that data nodes from OpenStreetMap are defined mostly by id, latitude and longitude, which is totally enough for painting 2D maps and standard routing queries. As the elevation is not added to OpenStreetMap data directly (and it is also not intended to be added to the OSM data base), a component was needed that parses both Nasa SRTM data as well as OSM data files in order to combine the data.

In the first version, we parsed the SRTM data directly and addied it to the nodes of the OSM-Graph directly. During one refactoring, we decided to integtrate Osmosis into the project in order to be able to read PBF files (another OpenStreetMap file format). During this integration we decided to separate the SRTM parsing into a separate module so that other people can make use of it as well. The plugin was open sourced some time ago at google code as the “osmosis-srtm-plugin” under an LGPL licence.

Relevant Links:

Drupal shows warnings after migrating to new server

Recently we migrated a Drupal managed website to a new server. – Everything was fine, just on some pages, some php-warnings showed up that did not show up on the original site.
After some investigation I checked php’s error_reporting setting and realized that the settings of the new new server were more sensitive than the old one.

The solution was pretty simple. As I definately didn’t want to start hacking around in Drupal, I just turned off showing the errors to the user:

Administer > Site configuration > Error reporting
(or in german:) Verwalten > Einstellungen > Error reporting
/admin/settings/error-reporting
Error Reporting: Write errors to the log (instead: Write errors to the log and to the screen).

Pretty easy.

TrafficMining Project goes open source

Quite some time ago I wrote about a little demo that was published at SIGMOD 2010 and SSTD 2011 (see post1 and post2).

The TrafficMining project could be described shortly as:

An academic framework for routing algorithms based on OpenStreetMapdata. Actually this framework is not intended to replace current routing applications but to provide an easy to use GUI for testing and developing new routing algorithms on real OpenStreetMap data.

Well, what makes this worth a post is the fact that we finally switched development over to GoogleCode with a discussion group at Google Groups.
GoogleCode has the major advantage of a Mercurial repository, an issue tracker, easy code reviews and an miproved possibility to contribute code. If you just want to follow the development, just join the google group or keep a bookmark to the project’s update feed.

By the way: the PAROS and MARiO downloads can be found there in the downloads section.

Sometimes AntiVirus Scanners just suck

Due to some very strange and non reproducible bugs I deleted my NetBeans profile to get back to a fresh post-install state. This mainly hat two very different results: A positive one and the one why I am writing this post. The positive one is/was that the mystery-bug disappeared.

After deleting the profile, NetBeans wants to update all its modules to the latest releases – which is okay.
The problem however was that NB never managed to download all the modules and just got stuck randomly between 0% and 100%. NB just said “downloading …” but the task manager showed 0 bytes/sec traffic. First I thought that the download/update servers might be overloaded. After 3 days of repeatedly trying – I was very douptful about that first guess. Finally I sent an email to the NetBeans mailinglist, if s.o else also experienced such a problem.

The first answer came in just 70min after my question saying “I had this problem and found that if I turn off AVG I could get the plugins.”.
Uhoh – I didn’t mention my AV scanner in my post – yet I am using AVG, too. And – surprise – after disabling resident shield, mail-scanner, link-scanner and identity protection – NB suddenly was able to update to the latest version. Unfortunately AVG never said that it was doing something of that kind!

So the lesson learned: If some really weird stuff is happening, try to disable the AV scanner – but don’t forget to re-enable it!

Update: same happens with Eclipse when downloading/installing plug-ins. – At least AVG is consistent with blocking Java-IDEs.