Reusing and Composing Components: Problems and Solutions

Building software from reusable components is considered important in reducing development costs. Object-oriented languages such as C++, Smalltalk and Java, however, are not capable of expressing certain aspects of applications in a composable way. Software engineers may experience difficulties in composing applications from components, for example if components implement code for multiple views, dynamic inheritance and synchronization [Aksit96]. If these aspects have to be programmed, then object-oriented languages may require a considerable amount of redefinition although this may not be intuitively necessary. To solve the composability problems, languages must be enhanced modularly without losing their basic characteristics. In addition, since more than one problem can be experienced for the same object, enhancements must be independent from each other. We have extended the conventional object-oriented model using the concept of composition-filters. Composition-filters can be attached to objects expressed for example in Smalltalk and Java. A number of different filter types have been defined, each addressing a certain concern. This paper first illustrates some practical problems and then introduces composition-filters solutions to overcome these problems