To assist Java programming educations, we have developed a Web-based Java Programming Learning Assistant System (JPLAS) along with the code writing problem which allows students to study writing source codes for assignments. As the object-oriented programming language, encapsulation, inheritance, and polymorphism are the three fundamental concepts that all the students are expected to master. In this paper, we propose the informative test code approach to the code writing problem, which helps students study them by describing the necessary information for the code. For evaluations, we generated informative test codes for 10 assignments, and then asked 10 students to write the source codes, where they completed the high-quality ones using the concepts.