Take some complicated calculation like
1) Calculate EMI
2) Calculate Tax
3) Calculate age of person
4) Find difference between any two given dates
5) Store Reminders of a person in a File. and whenever java file executed then read
reminders of that person.and display How many days to go.. etc...
If you want to to do a project in database management you ca do something like.... library management
student data/information management for a school,college....
a banking system
search at Google for topics.Here has much topics for mini project.
You will get much help from google.... http://www.sourcecodesworld.com/source/show.asp?ScriptID=867