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 file.
I think the quality of an error message can be categorized in one of the three categories:

  1. Reporting: The system just reports that there was an error.
    Example: IO Exception occured.
  2. 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
  3. 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

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!

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


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.


  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.

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
Error Reporting: Write errors to the log (instead: Write errors to the log and to the screen).

Pretty easy.

Finished my Posters for ICIP and MICCAI

Finally finished the posters for my publications:

F. Graf, H.-P. Kriegel, M. Schubert, S. Poelsterl, A. Cavallaro
2D Image Registration in CT Images using Radial Image Descriptors
In Medical Image Computing and Computer-Assisted Intervention (MICCAI), Toronto, Canada, 2011.


F. Graf, H.-P. Kriegel, M. Weiler
Robust Segmentation of Relevant Regions in Low Depth of Field Images
In Proceedings of the IEEE International Conference on Image Processing (ICIP), Brussels, Belgium, 2011.

Maximum Gain Round Trips with Cost Constraints

The idea is the following: Finding the shortest/fastes path from A to B is rather exploited. But if you start a hike, knowing that you want to spend 4 hours and then come back to the starting point. Then the problem suddenly starts to become a bit complex (NP-hard to be honest if you do not add any constraints).

We propose a solution to do this kind of search a bit more efficient. but don’t expect linear search time 😉 And – in contrast to quite some other research – we are operating on REAL data obtained from OpenStreetMap.


Searching for optimal ways in a network is an important task in multiple application areas such as social networks, co-citation graphs or road networks. In the majority of applications, each edge in a network is associated with a certain cost and an optimal way minimizes the cost while fulfilling a certain property, e.g connecting a start and a destination node. In this paper, we want to extend pure cost networks to so-called cost-gain networks. In this type of network, each edge is additionally associated with a certain gain. Thus, a way having a certain cost additionally provides a certain gain. In the following, we will discuss the problem of finding ways providing maximal gain while costing less than a certain budget. An application for this type of problem is the round trip problem of a traveler: Given a certain amount of time, which is the best round trip traversing the most scenic landscape or visiting the most important sights? In the following, we distinguish two cases of the problem. The first does not control any redundant edges and the second allows a more sophisticated handling of edges occurring more than once. To answer the maximum round trip queries on a given graph data set, we propose unidirectional and bidirectional search algorithms. Both types of algorithms are tested for the use case named above on real world spatial networks.


At our project site you can find:


  AUTHOR      = {F. Graf and H.-P. Kriegel and M. Schubert},
  TITLE       = {Maximum Gain Round Trips with Cost Constraints},
  INSTITUTION = {Institute for Informatics, Ludwig-Maximilians-University, Munich, Germany},
  YEAR        = {2011},
  LINK        = {}

Robust Segmentation of Relevant Regions in Low Depth of Field Images

Great, we got accepted (as a poster) on the ICIP 2011 with the paper “Robust Segmentation of Relevant Regions in Low Depth of Field Images”:

Low depth of field (DOF) is an important technique to emphasize the object of interest (OOI) within an image. When viewing a low depth of field image, the viewer implicitly segments the image into region of interest and non regions of interest which has major impact on the perception of the image. Thus, robust algorithms for the detection of the OOI in low DOF images provide valuable information for subsequent image processing and image retrieval. In this paper we propose a robust and parameterless algorithm for the fully automatic segmentation of low depth of field images. We compare our method with three similar methods and show the superior robustness even though our algorithm does not require any parameters to be set by hand. The experiments are conducted on a real world data set with high and low depth of field images. (Abstract from the paper)

The work is a result of a collaboration with Michael Weiler. We extended his Diploma thesis and produced an improved segmentation algorithm for Low Depth Of Field images. Compared to the other 3 competing algorithms, ours is a bit slower but at least it works. The other algorithms turned out to be extremely unstable and/or sensitive to parameters.

On the project site you can find

  • an online demo
  • the test images,
  • the masks
  • the NetBeans project including the full Java source code for our algorithm and the reimplementation of the comparison partners (of course we had to re-implement as we didn’t even get binaries – as usual)

So if you plan to do some image segmentation, just go there download the stuff and cite our work 😉

Fully automatic detection of the vertebrae in 2D CT images – the Talk

Yea finally I gave the talk for my Publication “Fully automatic detection of the vertebrae in 2D CT images” Paper 7962-11 at SPIE Medical Imaging 2011, Conference 7962 Image Processing (see index) in front of about 200 people.

Everything went fine. Just some nice questions right after the talk and some hints afterwards. Hey – some guys even remembered the talk 2 days later! 🙂

Thanks, SPIE Medical Imaging.

Bibliography Extension for MediaWiki

MediaWiki > Skins > Extension … a use case story

In mid 2009 we were asked to redesign our homepage to fit the corporate design of the LMU. During this time, I introduced and established MediaWiki as content management system for our website as it provided the needed flexibility, freedom and usability for our group.

One major issue was the list of publications which is very important for every researcher. In the old system, everyone maintained his own publication list manually. In parallel, all the publications were maintained in a central BibTex-file stored in CVS. Nice example for ‘Redundancy‘ – hey we’re a data base group – there shouldn’t be redundancies. So I wrote an extension in my spare time which parses a bibliography-file into a more convenient format. The result of such an automatically generated list can be seen for example on my publications list. – Well, finally I just put it online. Maybe you are a researcher or just maintinaing the website of some researcher(s) who already have a bib-File of their own publications and want to put your publications list online.

What this Extension can do / Features

The extension in action
The extension in action

This extension allows processing a central bibliography file in BibTeX format in order to create personalized publication pages for authors, projects or keywords. The BibTeX data can be stored in a file in the filesystem or in a special wiki article.

Implemented Features

  • The bibliography can be stored in a filesystem file or in a separate article of the wiki.
  • Multiple authors can share a single BibTeX source and have individual publication lists on their personal pages (=articles)
  • Filtering can be done on all attributes of the bibtex file/article.
  • Filters can be combined: for example, all papers of year=2010 AND author=xy AND keyword=xyz
  • Supports optional bibtex entries (“pages =100–110” may be entered in one bibtex entry but not in another – but if the entry is present, it should be formated as “p. 100-110”. If the attribute is not there, “p. ” should also not appear)
  • Different bibliography-types (article, book, inproceeding, …) use different styles according to the mandatory and optional fields.
  • @unpublished entries will be ignored
  • Supports @String-replacement as it is can be done by BibTeX
  • Automatically adds a separator between entries of different years
  • Provide additional links for each BibTeX entry (like PDFs or links to articles with further informations)
  • Author names can be linked automatically to pre defined wiki articles


Just visit my How-To page, to see what the extension can do.
You can download the extension by using the link in the download section.

Maybe you can just drop me a line if you find the extension useful.