Improve Java Experience
I'm going to go ahead an assume that it's a Java 101 type of exam.

The best way to go about studying for it, in my opinion, is to practice applying the basics. Things such as Data Types, Conditional Statements, using Arrays, etc. A good place that has stuff like this is CodingBat as it gives you answers too. You could even just have a read through a beginner's Java book (a quick google search will bring up loads which you can find a PDF easily).

Then if you want to work on general problems that encompass everything you could do the problems at places like HackerRank (Which is also good practice for coding interviews as many companies use this) or the easy problems at reddit.com/r/dailyprogrammer. There are tonnes of other resources out there but if you would like some more just comment on this.

Good luck in your exam and I hope this helps you a little bit.