Hello, how would i go about doing a saveData() and loadData() method in java?

Hello, how would i go about doing a saveData() and loadData() method in java? So there is initially a menu which the user can pick from whether to input data or view data,

eg. add patient id, add observations etc and then there is also an option to save the data to a text file or load from a text file. I've outputted all the info to the screen but i just want to know how i can then save all the data when i do the save choice from the menu. Thank you!