If the literal answer to your question, then any accounting system, such as a telephone directory. The data stored in the file (or not to store at all).
If you want advice, then:
Learn core java training with 8 to 17:00-this month max. If you want to become a professional (ie get paid for writing java programs) and you are young-then read a little bit about the questions in the interview and get a job Joon. If you are over 30 then:
1. java-is the knowledge of English at level of reading without a dictionary-be sure. Write and verbally communicate in English is highly desirable. I don't know -- teach.
2. java-is server software for accounting systems. For example, the program in the Bank. Until you can start to think how to write an application that will never come down, and if you fall, the log will immediately clear what was wrong-you way further study is closed. That is, in the conventional phone book when you add a record in the database, you can provide all. Starting from the fact that the base had fallen, ending with the fact that they put 100 people a day give you an incorrect value.
well, quite off topic, but you probably know. Narrow core Java knowledge is not really needed anyone. Need frameworks, e.g. hibernate and dozens of them.