This week's book giveaway is in the OO, Patterns, UML and Refactoring forum. We're giving away four copies of Refactoring for Software Design Smells: Managing Technical Debt and have Girish Suryanarayana, Ganesh Samarthyam & Tushar Sharma on-line! See this thread for details.
There are several problems with the code... ... including the fact that you're not initializing testNumber ... which will cause all values you insert to compare as "==" ... which won't work with a "Set" (where all values must be unique...)
But the real problem is shown in this traceback (which you probably should have cut/pasted along with the code):
C:\temp>java test Exception in thread "main" java.lang.ClassCastException: test cannot be cast to java.lang.Comparable at java.util.TreeMap.put(Unknown Source) at java.util.TreeSet.add(Unknown Source) at test.main(test.java:10)