L11Q2 java class file

import java.util.ArrayList;
/**
 * /
 */
public class L11Q2
{
    public static void main(String[] args){
        Subject math = new Subject ("math","Mathematics");
        Subject stat = new Subject ("stat","Statistics");

        Course data = new Course();
        Course stats = new Course();
        Course top = new Course();
        Course calc = new Course();
        data.setCourseId("222");
        data.setCourseName("Data Analysis");
        data.setCreditHours("3");
        stats.setCourseId("123");
        stats.setCourseName("Statistics1");
        stats.setCreditHours("3");
        top.setCourseId("102");
        top.setCourseName("Topology");
        top.setCreditHours("3");
        calc.setCourseId("101");
        calc.setCourseName("Calclus");
        calc.setCreditHours("6");

        //How do i link courses and subjects

        //Display each subject with the coures they offer
        System.out.println("Subject Math Courses Offered: " + math.getCourses());
        System.out.println("Subject Stat Courses Offered: " + stat.getCourses());
        // im getting result "null"
    }
}