The program works. Each file has a meaningful public class name that is the same name as your file. There is at least one other comment in each main method. Consider what we will look for in this particular program Each class is in its own file. You have a class named StartOneAccount with one main method for setting up the GUI and connecting everything.
All instance variables are private.
You did not use instance variables when local variables were appropriate. If a variable is needed in only one method, declare it there. Do not declare it outside of the methods. The Amount field starts off blank. The toString version of the single BankAccount is always visible.