So, I have my main class that calls private Secondary secondary = new Secondary(); when it runs. In the Secondary class, at the top I have code that says private Main main = new Main();.
How will I be able to use all of the methods and variables from the Secondary class and vice versa without causing a stack overflow error?
Note: they are not in the constructor
Your Main class is creating a Secondary instance, which is creating a Main instance..., and this is causing the stack overflow error.
I think you just want the objects to refer to each other, so don't create the other class's new instance in the constructor. Declare references as instance variables, and use setter methods to store existing references to objects of the other type.
You could use the builder pattern to handle safe (non-recursive) initialization of your instance fields (via setters).
来源：https://stackoverflow.com/questions/13203992/create-objects-without-causing-a-stack-overflow-error