It is written in the jpcap documentation that packet class is a root class of the all the packets captured by Jpcap.So, by root class what do they mean ?They want to say that it will have header and data from the datalink layer till application layer?Its not clear to me because in every documentation i have found only this one line is written that it is the root class.
Its the root class cause other classes like EthernetPacket , ICMP, IP,TCP, UDP etc are its child . for getting information about other classe ,you have to convert your packet class to others.