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In this post, I briefly discuss the approach we developed for Google Hashcode 2016.
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Last week, a colleague and I participated in Google’s Hash Code. The idea of the event is simple: give small teams of programmers a hard task and give them too little time to solve it. See what they can come up with.
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For our master’s project, we include visualization of control flow graphs of Java bytecode as a user interface feature. When discussing the feature, a question quickly became apparent: are control flow graphs (CFG) planar? If so, we expect drawing them to be relatively easy. If not, however, we can abandon hope for being able to always draw nice graphs.
Obviously, unconditional jumps can cause arbitrarily nasty CFGs; that is not surprising as goto is generally to be considered harmful. We agreed, however, that we should rather consider basic blocks of actual Java rather than arbitrary bytecode, as it is Java code we want to talk about. Sadly, even if we disregard labeled break just to be save, Java CFGs are not planar in general. The examples we found suggest that this is true for most procedural and object oriented languages, too.
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While coding away on our master’s project, my team and I faced a problem. In our setting, users provide Java byte code to a client application which instruments it and passes the resulting JAR archive to remote servers via RMI. The servers execute this code in separate processes and feed it inputs values fetched from the client, again via RMI. Now, this can be a problem as users may provide their own implementations for those input values. Both client and separately started processes have, of course, access to these implementations, but servers may not.
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Last week, Barbara Liskov visited Kaiserslautern and gave her talk On the Power of Abstraction. I had known her from the substitution principle named after herShe did not name it, of course. Apparently, she received an email in the 90’s by somebody asking her whether he got her principle right, surprising her. She had not known that the principle had borne her name for years in the community. and taught to every computer scientist and programmer, but she is far more distinguished than that: Barbara Liskov is an ACM Fellow, holds the ACM Turing Award, the IEEE John von Neumann Medal, and a couple of other awards, telltale of her influence on the field.
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