I have created a java random password generator. Now I can't complete it as I don't know some of the coding part ? Can anyone help me complete it?

I have created a java random password generator for windows pc. Now I can't complete it as I don't know some of the coding part ? Can anyone help me complete it ?The link to my incomplete code is given below. Those who will help me in this project will get their names printed in the contribution section of this application and also on it's website ? Link: https://drive.google.com/drive/folders/17-Ep-zjsequBBIEu5hQy8TvwOQDyOeUk?usp=sharing. Please help me.