NoClassDef exceptions are really only thrown for the fact the class simply doesn't exist.
Are you running the application through an IDE? If so, make sure the jar is in the classpath. If not, make sure you're including it in the command prompt.
java -classpath thirdpartyjar.jar mainclass.java
There can be no reason for this exception other than the class doesn't exist or the lib is in the wrong place.
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