You need to break your problem down. Before you go into using ArrayList or any other container, you need to work with extracting the information from your FFT related to your frequencies of interest. Are you confident you can do this? Are you able to do it now? it is not clear from the code that you posted if you are able to do this core task in your challenge.
To do this first part, go back to the basic program in minim that extracts the frequency bands from your sample sound file. From this code, you need to generate the code that allows you to pick the BASS and the HIGH from the F spectrum. How are you choosing these values? Do you have a threshold value that separates low from the high frequency? After you get this code done, we can go to a second step which is saving these value pairs when you press a key. A third task is Saving the value pairs in a container like an ArrayList. Finally, one works in the visualization part.
Kf