Packet Sniffer? (Use Jpcap or npcap,.(libraries if you are going to use java. READ the Docs.)). Doing this using python is alot easier though. 

Port Scanner? (This one is easier, requires so much less work.)

and there might be another one?

make an app that pings a host or a network in multi-thread multiple times (user able to set the number of pings per second). This could act as a Dos attack, if you can deploy your app on multiple pc and use all of them to ping a particular host (let's say 10k times a second.). pretty sure you can expect good damage rate. you can either be stealthy about your attack or just use normal ping packets (which is usually filtered by the firewalls.)

:)

If i get more ideas, I will tell you.

good luck