OBJECT-ORIENTED DESIGN EVALUATION USING ALGEBRAIC GRAPH THEORY Spiros Xanthos, Alexander Chatzigeorgiou and George Stephanides Department of Applied Informatics,

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OBJECT-ORIENTED DESIGN EVALUATION USING ALGEBRAIC GRAPH THEORY Spiros Xanthos, Alexander Chatzigeorgiou and George Stephanides Department of Applied Informatics, University of Macedonia, Thessaloniki, Greece Contact: GOALS 1.To identify and quantify heavily loaded portions of an object- oriented design by algorithmic means 2.To identify dense regions of classes in the system by spectral graph partitioning “GOD” CLASSES The outcome of the analysis/design phase is often one or more "God” objects, which perform most of the work in the system. Clearly, such a solution is not managing complexity any better than procedural programming and implies a poorly designed model. Paraphrasing Kleinberg [1]: A class c holds a central role in a model: a. if it receives many messages from other central classes (candidate of a good authority) b. if it sends many messages to other classes which are also central (candidate of a good hub) LINK ANALYSIS The set of all classes can be represented as a directed graph G=(V, E) where vertices correspond to the classes and a directed edge indicates an association between classes p and q. Each edge is annotated with the number of discrete messages sent to the same direction (m p,q ). Given a set T of associated classes, the aim is to find the authority and hub weights (a p, h p ), associated with each class. Classes with higher authority and hub weights are viewed as classes having a more important role in the model. Employing the Power Method of Linear Algebra it can be shown that: If A denotes the adjacency matrix of the graph G in the model under study, then the authority and hub weights are given by the elements of the normalized principal eigenvector of A T A and A A T, respectively. References [1] J. M. Kleinberg, “Authoritative Sources in a Hyperlinked Environment”, Journal of the ACM, vol. 46, issue 5, Sep. 1999, pp [2] M. Fiedler, “A property of eigenvectors of nonnegative symmetric matrices and its application to graph theory”, Czechoslovak Mathematical Journal, 25 (1975), pp EXAMPLES In the following design, other measures (i.e. in-out degree) would not be able to differentiate between the role of the central and peripheral classes. Link analysis clearly identifies C3 as the most heavily loaded class: Colors show the distribution of workload. Bright red indicates heavily loaded classes. Principal eigenvectors of A T A and AA T DENSE COMMUNITIES The objective is to group classes in such a way that the inter-group coupling is minimized and the intra-group coupling is maximized. Dense communities of classes, might imply relevance of functionality and thus indicate a portion of the overall design that has a distinct purpose. The second eigenvector of the Laplacian matrix of G provides a natural way for bi-partitioning the underlying graph: Positive and negative entries in the second eigenvector correspond to two groups of classes, which, if partitioned, minimize the coupling between the partitions [2].