This is related to programming with one of the Java APIs:

The CLASSPATH is the environment variable, which tells the Java VM, where to search for class files. We often investigate errors, which are related to incorrect settings of this environment variable. Mostly the error does not suggest that the problem is the CLASSPATH setting. So therefore here are some simple things, you should check after you encounter an error, which you can’t explain, and before contacting us:

• Make sure, the CLASSPATH contains all *.jar files from the Java API, you are using!
  
• Make sure, all these *.jar files are from the same release!
  
• Do not mix *.jar files from different Java APIs!