Every class file contains the full information about the class name, package and everything needed to instantiate the class... it even includes the source class

Every class file contains the full information about the class name, package and everything needed to instantiate the class... it even includes the source class file name usually and other attributes like statement line numbers. However, for performance reasons, the JVM loads classes lazily and will only try to find classes in the directory it would be expected to be (either in the directories included in the classpth or in jars). Because you can write your own class loader, you can implement something different (like OSGi does for example - it has much more metadata so it can even load multiple classes with the same name from different jars) but this is the default behaviour.