This research investigates how object-oriented inheritance is actually used in practice. The aim is to close the gap between inheritance guidance and inheritance practice. It is based on detailed analyses of 2440 inheritance hierarchies drawn from 14 open-source systems. The original contributions made by this paper concern pragmatic assessment of inheritance hierarchy design quality. The findings show that inheritance is very widely used but that most of the usage patterns that occur in practice are simple in structure. They are so simple that they may not require much inheritance-specific design consideration. On the other hand, the majority of classes defined using inheritance actually appear within a relatively small number of large, complex hierarchies. While some of these large hierarchies appear to have a consistent structure, often based on a problem domain model or a design pattern, others do not. Another contribution is that the quality of hierarchies, especially the large problematic ones, may be assessed in practice based on size, shape, and the definition and invocation of novel methods - all properties that can be detected automatically.

Design quality is vital if software is to be maintainable. What practices do developers actually use to achieve design quality in their day-to-day work and which of these do they find most useful? To discover the extent to which practitioners concern themselves with object-oriented design quality and the approaches used when determining quality in practice, a questionnaire survey of 102 software practitioners, approximately half from the UK and the remainder from elsewhere around the world was used. Individual and peer experience are major contributors to design quality. Classic design guidelines, well-known lower level practices, tools and metrics all can also contribute positively to design quality. There is a potential relationship between testing practices and design quality. Inexperience, time pressures, novel problems, novel technology, and imprecise or changing requirements may have a negative impact on quality. Respondents with most experience are more confident in their design decisions, place more value on reviews by team leads and are more likely to rate design quality as very important. For practitioners, these results identify the techniques and tools that other practitioners find effective. For researchers, the results highlight a need for more work investigating the role of experience in the design process and the contribution experience makes to quality. There is also the potential for more in-depth studies of how practitioners are actually using design guidance, including Clean Code. Lastly, the potential relationship between testing practices and design quality merits further investigation.

Given two designs that meet the same specification -- to what extent can we use objective measures to assert that either of the designs is of superior quality? This research focuses on design quality at the class interaction-level and the extent to which we can compare the quality of software designs - with an aim of being able to identify exemplars of 'good design' and give practical advice on design problems in both industrial and educational settings. An initial pilot study examining "program to an interface" use in open source systems has identified that any general notion of design quality must be applicable across a wide range of systems and architectures.