Recently, the objected-oriented programming language Java has been used in many practical systems including enterprise servers, smart phones, and embedded systems due to its high reliability and portability. To enhance effects of Java programming educations, we have developed a Web-based Java Programming Learning Assistant System (JPLAS). JPLAS adopts the Test-Driven Development (TDD) method for automatic testing of source codes written by students on the server to assist their self-learning. In JPLAS, a teacher needs to register each Java programming assignment with a statement, a reference source code, and a test code. Then, students should write source codes by referring this statement and the test code so that their source codes can be tested automatically at the server by a testing software tool called JUnit with the test code. Unfortunately, the current JPLAS is not friendly to students who have difficulty in reading the log message from JUnit that contains the information on the errors in the source code. In this paper, we implement the error code highlighting function to help such students to find erroneous lines in the code by highlighting them graphically. We evaluate the effectiveness of this function through an experiment for 42 students who are currently taking the Java programming class.