How to create a for statement to handle my Exception

I had previously created a program that contained a customerList that added courses to each customer via their courseList. Now I have to modify the program (having a new list of customers) to throw an exception called CustomerNotEnrolledException if one the customers are not enrolled in any course.

I throw the exception from a createInvoice method in my Customer class, and handle it in the test class. My question is:

How will I write a for loop to check for these courses within each customer.

The two arrays that I have declared earlier are:

ArrayList<Course> courseList = new ArrayList<Course>();
ArrayList<Customer> customerList = new ArrayList<Customer>();