public class Class {
    private final List<Student> students;

    /**
     * Throws IllegalArgumentException if any of the students are null
     */
    public Class(List<Student> students) {
        for (Student student : students) {
            if (student == null) {
                throw new IllegalArgumentException();
            }
        }
        this.students = students;
    }

    public List<Student> getStudents() {
        return students;
    }

    public void printClassRoster() {
        for (Student student : students) {
            // there shouldn't be any null students here
            System.out.println(student.getName() + " " + student.getGPA());
        }
    }
}

/**
 * Write client code using Class and Student that will cause printClassRoster to throw a NullPointerException.
 *
 * One possible solution:
 *  public static void badClient() {
 *      List<Student> students = ...; //Initialize a list.
 *      Class cse373 = new Class(students);
 *      class.printClassRoster(); // This is fine
 *      students.add(null);
 *      class.printClassRoster(); // NullPointerException
 *  }
 *
 * What changes need to be made to these classes to preserve abstractions for the entire Class class?
 * Note you might have to change Student too.
 *
 * Solution:
 *   Make a copy of the given list in the constructor
 *   Make a copy of the list in getStudents to return
 *   Make the Student fields private
 *   Either make the Student fields final or also make copies of each individual Student in the Class constructor and getStudents.
 */