For me it was migrating the unit tests for a whole project from JUnit 4 to 5. Reading and understanding the code, transforming it into a newer version of the fr

For me it was migrating the unit tests for a whole project from JUnit 4 to 5. Reading and understanding the code, transforming it into a newer version of the framework and doing some cleanup / optimization while at it was proof to me that my skills were also applicable to "large", "real" projects.