理解Java代码中使用堆栈的后缀表达式求值

I was given a snippet of code to decipher, explain and offer any recommendations for improvement. I was told it works and we can't run the code to test it. I pretty much understand it but just need to run it by someone to make sure what I do understand about it is correct and please get any help with explaining what I don't understand.I have been doing a ton of research and still have some questions.I was given a snippet of code to decipher, expl