@iSyriux since you asked, I'd really start by reading java tutorials (not watching, who wants to waste 1 hour watching a video when you can read it in 5 minutes?)
There're many sources, I referred to geeks for geeks sometimes, https://www.tutorialspoint.com/java/ is also one that I learned java from and alot more, uncle google has the answers
Also btw, using a class name such as custom_model isnt really good, would be better if you call it Custom_Model atleast, if anyone was taking a quick look at your code they might think it's a variable instead of a class.