To enhance educational effects of Java programming by assisting self-studies of students and reducing teaching loads of teachers, we have proposed a Web-based Java programming learning system using the test-driven development method. In this system, a teacher should register Java programming assignments with statements, model source codes, and test codes using a Web browser. Then, a student can submit a test code and an answer source code for each assignment, where both codes are tested automatically by a testing tool called Junit at the server. Unfortunately, the current system cannot identify an incomplete test code that does not contain the complete test procedures if it has no grammatical error. In this paper, we introduce a code coverage measurement tool called Cobertura to detect such a test code by measuring the coverage rate when the submitted test code tests the model source code. We evaluate the effectiveness of our improved system through experiments with two simple assignments to 11 students who have studied Java.