Blessing or Curse? Investigating Test Code Maintenance Through Inheritance and Interface

Since the advent of object-oriented programming languages, inheritance and interface have been fundamental concepts in software design principles, facilitating code reuse and extensibility in software systems. Despite their potential benefits, inheritance, and interface remain underexplored in software test code. Currently, there is a limited established standard for how inheritance and interface may impact test reusability, extensibility, and maintainability, nor for understanding the potential design challenges that may arise from improper usage. Addressing these research gaps is crucial for optimizing test maintainability and software quality. In this paper, we address this gap in empirical research by conducting the first comprehensive study on the prevalence and maintenance of inheritance and interface within test code. To accomplish this goal, we use RefactoringMiner's AST differencing API to detect inheritance and interface changes in modified test classes within the software evolution commit history by studying 12 open-source Java systems. Our key findings are as follows: (1) Among the 23,651 commits that modify test classes, 4,429 (18%) involve changes to their inheritance relationships, whereas a significantly smaller subset, 233 (1%), pertain to changes in their interface relationships. (2) 59.5% of test classes already incorporate inheritance when initially created, while 40% of test classes incorporate interfaces. (3) We manually categorized the use of inheritance and interfaces and their impact on test maintainability to provide valuable insights for developers. In summary, this study takes the first step in exploring how the use of inheritance and interfaces in test code affects software reusability and extensibility, offering meaningful insights for both developers and researchers