To support novice students in learning Java programming, we have implemented the web-based Java Programming Learning Assistant System (JPLAS). In JPLAS, one of the implemented exercise problems is the Code Writing Problem (CWP), where students are tasked with crafting a source code capable of passing provided test cases. The accuracy of the answer code is verified by executing it on JUnit. Previously, we generated seven CWP instances accompanying with test codes, focusing on six fundamental Object-Oriented Programming (OOP) concepts. These assignments were intended for use in a Java programming course at Okayama University, where OOP is a crucial topic. Despite their relevance, these assignments were not assigned to students for completion. In this paper, we present an analysis of the solution results obtained from the CWP instances. Our findings indicate that the students demonstrated a comprehensive understanding of basic OOP concepts, as the majority of them successfully solved all the generated instances and achieved a 100% correct rate. Only three students did not complete all the instances. Based on these results, we conclude that the proposed approach is valid and effective in facilitating student learning of OOP concepts.