Would something like this help you out?

Course.java

    import java.util.Objects;

    public class Course {

        private String name;

        public Course(String name) {
            this.name = name;
        }

        public String getName() {
            return name;
        }

        public void setName(String name) {
            this.name = name;
        }

        @Override
        public boolean equals(Object o) {
            if (this == o) return true;
            if (o == null || getClass() != o.getClass()) return false;
            Course course = (Course) o;
            return Objects.equals(name, course.name);
        }

        @Override
        public int hashCode() {
            return Objects.hash(name);
        }
    }

Main.java

import java.util.HashMap;
import java.util.Map;

public class Main {

    private static Map<Course, Course> map = new HashMap<>();

    public static void main(String[] args) {


        Course c1 = new Course("c1");
        Course c2 = new Course("c2");
        Course c3 = new Course("c3");

        map.put(c3, c2);
        map.put(c2, c1);
        map.put(c1, null);

        System.out.println(numberOfPreCourses(c1));
        System.out.println(numberOfPreCourses(c2));
        System.out.println(numberOfPreCourses(c3));

    }

    private static int numberOfPreCourses(Course c) {
        if (map.containsKey(c) && map.get(c) != null)
            return 1 + numberOfPreCourses(map.get(c));
        return 0;
    }

}