Difference between ClassNotFound exception and NoClassDefinition

ClassNotFoundException comes when JVM tries to load a class at runtime dynamically means you give the name of class at runtime and then JVM tries to load it and if that class is not found in classpath it throws ClassNotFoundException.  ClassNotFoundException comes on Runtime when requested class is not available in classpath and mainly due to call to Class.forName () or Classloader.loadClass () or ClassLoader.findSystemClass ().

While in case of NoClassDefFoundError the problematic class was present during Compile time and that's why program was successfully compile but not available during runtime by any reason.

For More:- http://javarevisited.blogspot.in/2011/08/classnotfoundexception-in-java-example.html