Background: Since Google introduced Kotlin as an official programming language for developing Android apps in 2017, Kotlin has gained widespread adoption in Android development. The interoperability of Java and Kotlin's design nature allows them to coexist and interact with each other smoothly within a project. Aims: However, there is limited research on how Java and Kotlin interact with each other in real-world projects and what challenges are faced during these interactions. The answers to these questions are key to understanding these kinds of cross-language software systems. Methods: In this paper, we implemented a tool named DependExtractor, which can extract 11 kinds of Kotlin-Java dependencies, and conducted an empirical study of 23 Kotlin-Java real-world projects with 3,227 Java and 8,630 Kotlin source files. Results: Our findings revealed that Java and Kotlin frequently interact with each other in these cross-language projects, with access and call dependency types being the most dominant. Compared to files interacting with other files in the same language, Java/Kotlin source files, which participate in the cross-language interactions, undergo more commits. Additionally, among all Kotlin-Java problematic interactions, we identified seven common mistakes, along with their fixing strategies. Conclusions: The findings of this study can help developers understand and address the challenges in Kotlin-Java projects.

翻译：背景：自2017年谷歌将Kotlin作为开发安卓应用的官方编程语言以来，Kotlin在安卓开发中得到了广泛应用。Java与Kotlin的设计特性使其具备互操作性，允许两者在同一项目中顺畅共存与交互。目的：然而，关于Java和Kotlin在实际项目中如何交互，及这些交互过程中面临哪些挑战的研究尚不充分。解答这些问题对于理解这类跨语言软件系统至关重要。方法：本文实现了一个名为DependExtractor的工具，可提取11种Kotlin-Java依赖关系，并对23个实际Kotlin-Java项目（包含3,227个Java源文件和8,630个Kotlin源文件）进行了实证研究。结果：研究发现，在这些跨语言项目中，Java与Kotlin频繁交互，其中访问依赖和调用依赖类型最为常见。与同语言文件间的交互相比，参与跨语言交互的Java/Kotlin源文件经历了更多的提交次数。此外，在所有Kotlin-Java问题性交互中，我们识别出七类常见错误及其修复策略。结论：本研究结果有助于开发者理解并应对Kotlin-Java项目中的挑战。