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This empirical research was undertaken as part of a multi-method programme of research to investigate unsupported claims made of object-oriented technology. A series of subject-based laboratory experiments, including an internal replication, tested the effect of inheritance depth on the maintainability of object-oriented software. Subjects were timed performing identical maintenance tasks on object-oriented software with a hierarchy of three levels of inheritance depth and equivalent object-based software with no inheritance. This was then replicated with more experienced subjects. In a second experiment of similar design, subjects were timed performing identical maintenance tasks on object-oriented software with a hierarchy of five levels of inheritance depth and the equivalent object-based software. The collected data showed that subjects maintaining object-oriented software with three levels of inheritance depth performed the maintenance tasks significantly quicker than those maintaining equivalent object-based software with no inheritance. In contrast, subjects maintaining the object-oriented software with five levels of inheritance depth took longer, on average, than the subjects maintaining the equivalent object-based software (although statistical significance was not obtained). Subjects' source code solutions and debriefing questionnaires provided some evidence suggesting subjects began to experience difficulties with the deeper inheritance hierarchy. It is not at all obvious that object-oriented software is going to be more maintainable in the long run. These findings are sufficiently important that attempts to verify the results should be made by independent researchers

This paper describes an investigation into the viability of mental models used by novice programmers at the end of a first year Java programming course. The qualitative findings identify the range of mental models of value and reference assignment held by the participants. The quantitative analysis reveals that approximately one third of students held non-viable mental models of value assignment and only 17% of students held a viable mental model of reference assignment. Further, in terms of a comparison between the participants' mental models and their performance in incourse assessments and final examination, it was found that students with viable mental models performed significantly better than those with non-viable models. These findings are used to propose a more "constructivist" approach to teaching programming based on the integration of "cognitive conflict" and program visualisation.

Current software visualisation tools do not address the full range of software comprehension requirements. This paper proposes a novel software visualisation model for supporting object-oriented software comprehension that is intended to address the shortcomings of existing tools. We discuss the previous work that prompted us to develop this model. An initial model is then presented, based on multiple levels of abstraction, multiple perspectives of the software system, and the integration of statically and dynamically extracted information. We review the evaluation tasks used in our previous work and those from the software visualisation and comprehension literature to produce a refined set of evaluation tasks. We then use these tasks to perform an initial assessment of the proposed model. The refined model is then defined more formally. Finally, a concrete example of the use of the model to generate abstraction hierarchies is discussed. We conclude that a visualisation model incorporating a hierarchy of interrelated abstraction levels, combined with structural and behavioural perspectives of the software, will provide effective support for software comprehension.

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We provide motivation for researchers to replicate experiments in software engineering. The ideology of replication is discussed. We address the question: Is an experiment worth repeating? The current lack of replication studies is highlighted. We make clear that exact replication is unattainable and we draw on our first experience of performing an external replication. To categorise various kinds of replication, we propose a simple extension to Basili et al.'s framework for experimentation in software engineering. We present guidance as to the level of reported detail required to enable others perform a replication. Our conclusion is that there is only one route for empirical software engineering to follow: to make available laboratory packages of experimental materials to facilitate internal and external replications, especially the latter, which have greater confirming power.

The teaching of introductory computer programming seems far from successful, with many first year students performing more poorly than expected. One possible reason for this is that novices hold 'non-viable' mental models (internal explanations of how something works) of key programming concepts which then cause misconceptions and difficulties. An initial study investigated the apparent viability of novices' models of fundamental programming concepts, focusing on value and reference assignment. This revealed that many students appeared to hold 'non-viable' mental models of these key concepts and that those students who appeared to hold viable mental models performed significantly better in programming tasks than those with non-viable models. To address this, a teaching model integrating cognitive conflict and program visualisation is proposed. A series of studies found that this teaching model is potentially effective in enhancing engagement with learning materials and may therefore help novice programmers develop a better understanding of key concepts

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