A software component must be tested every time it is reused, to guarantee the quality of both the component itself and the system in which it is to be integrated. So how to increase testability of component has become a key technology in the software engineering community. This paper introduces a method to increase component testability. Firstly we analyze the meanings of component testability and the effective ways to increase testability. Then we give some definitions on component coupling testing criterion. And we further give the definitions of DU-I(Definition-Use Information) and OP-Vs (Observation-Point Values). Base on these, we introduce a definition-use table, which includes DU-I and OP-Vs item, to help component testers understanding and observing the component better. Then a framework of testable component based on above DU-table is given. These facilities provide ways to detect errors, to observe state variables by observation-points based monitor mechanism. And we adopt coupling-based testing using information DU-table provided. Lastly, we applied the method to our application software developed before, and generate some test cases. And our method is compared with Orso method and Kan method using the same example, presenting the comparison results. The relevant results illustrate the validity of our method, effectively generating test cases and killing more mutants.
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