Voxxed Days Luxembourg 2023 Room: Linux Type: Conference Do you know what is inside of your class files? Are you ready to bet? Let's have fun by diving into bytecode pattern matching and explore how different compilers (Java/Kotlin) and their different versions, from the old ones up to the most recent, transform your source code. Our dive will be based on details of the implementation of the most popular open-source code coverage tool for Java – JaCoCo ( https://www.jacoco.org/ ), which finds in bytecode and filters out various compiler generated artifacts, which otherwise require unnecessary and sometimes impossible tricks to have full code coverage.