JTree error in compiling with jdk 5

Hello:

I have just installed jdk 5. On the very first trial compilation of my application I received the following error:

"S3i2BusinessDB.java": cannot access javax.swing.JTree,bad class file: C:\JBuilder9\Java\jdk1.5.0_01\jre\lib\rt.jar\javax\swing\JTree.class,class file has wrong version 49.0, should be 48.0,Please remove or make sure it appears in the correct subdirectory of the classpath. at line 5, column 20

What does that mean and what I should do to correct the problem?

Obviously I have not changed anything after running the downloaded file.

Thank you,

Alex Bogomolny

[630 byte] By [AlexBo] at [2007-11-25 18:52:50]