Currently, Java programming has been educated in a lot of universities to students in IT departments for teaching object-oriented programming. To enhance it, we have developed the Web-based Java programming learning assistant system (JPLAS). JPLAS offers the code writing problem that asks a student to implement a source code that passes the given test code on JUnit. However, the correctness of the answer code from a student in satisfying the assignment intensions depends on the quality of the test code. If it is insufficient, the answer code can be passed, although it does not implement them. In this paper, we discuss four insufficient test codes in current JPLAS and their improvements. The effectiveness of the improvements is confirmed from the answer codes by improved test codes.