JarScan is a handy utility (available as a client and online tool) which can help locate a java class or package within jars and zip files (libraries). This is especially useful when debugging java.lang.ClassNotFoundException and java.lang.NoClassDefFoundError exceptions.
A screenshot of JarScan usage is given below:
Refer this example which shows how JarScan was used to find the location of the MulticastTest class.
A while ago, when I had a requirement to locate a Java class file, I simply used jar –tvf and grep. Although that worked, I would henceforth prefer to use JarScan, as it provides a good output, it is fast and it is platform independent.