Loading classes based on name and superclass

In games that i have modded that run on java a class that starts with mod_*** is loaded into the system and if it extends another class a method is called i was wondering how they iterated through all the classes in the package and then created objects of all the right classes plz help if this post is not clear plz tell me and ill try to specify further

my java tutor told me their was some method in the file class to do this but i couldnt find any