Random access a value from Java HashMap, when using a custom class object as key for HashMap?

Question

I am using a custom class object as the key for a HashMap. In this class definition, I have overridden the equals() and hashCode() methods.

public class TimeTableDataModel {

    Map <Course, List <Timings>> tm; 

    TimeTableDataModel() {
        tm = new HashMap<>();
    }

    void addCourseItem(Course course) {
        tm.put(course, new ArrayList<Timings>());
    }

    void addNewTimeTableItem(Course course, Timings newTiming) {
        List <Timings> t;
        if(!tm.containsKey(course)) {
            addCourseItem(course);
        }
        t = tm.get(course);
        t.add(newTiming);
        tm.put(course, t);
    }

    public static final class Course {
        private final String courseCode;
        private final String courseName;
        private final String section;
        private final String group;

        Course(String code, String courseName, String section, String group) {
            this.courseCode = code;
            this.courseName = courseName;
            this.section = section;
            this.group = group;
        }

        public String getCourseCode() { return courseCode; }
        public String getCourseName() { return courseName; }
        public String getSection() { return section; }
        public String getGroup() { return group; }

        @Override
        public boolean equals(Object obj) {
            if (this == obj) {
                return true;
            }
            if (!(obj instanceof Course)) {
                return false;
            }
            Course otherObj = (Course) obj;
            return Objects.equals(courseCode,otherObj.courseCode) 
                && Objects.equals(courseName, otherObj.courseName) 
                && Objects.equals(section, otherObj.section)
                && Objects.equals(group, otherObj.group);
        }

        @Override
        public int hashCode() {
            return Objects.hash(courseCode, courseName, section, group);
        }       
    }

    public static class Timings {
        String time;
        String day;
        String room;

        Timings(String time, String day) {
            setTime(time);
            setDay(day);
        }

        public String getTime() { return time; }
        public String getday() { return day; }

        public void setTime(String time) { this.time = time; }
        public void setDay(String day){this.day = day;}
    }
}

In above code I have created Course class to be used as the key for the HashMap and using a List<Timings> for values. What I intend is to get a List of timings when a Course is passed to hm.get(course). So far I can get a keyset then sequentially get values for each course.

for(Course c : timetable.tm.keySet()) {
    System.out.println(c.getCourseCode() + " " + c.getCourseName());
    for(Timings t : timetable.tm.get(c)) {
        System.out.println(t.time + " " +t.room + " "+ t.day);
    }           
};

Here's the code that populates the HashMap

static TimeTableDataModel timetable = new TimeTableDataModel();

Course course = new Course(courseCode,null,section,group);
Timings dt = new Timings(time, getDayOfWeek(i));
dt.room  = roomNo;
timetable.addNewTimeTableItem(course, dt);

So to get the timings for a particular course I have to traverse the whole HashMap until the desired course Key is found. What I want is a way to distinguish between each course object contained in the HashMap Key, so I can get Timings for any random course without traversing the whole KeySet.

Thanks in advance. Please ask if somethings is unclear in code

Answer 1

Problem what I see here is

 if(!tm.containsKey(course)){
        addCourseItem(course);
    }

and

if (this == obj) {
            return true;
        }

because you are comparing the object. Since both are same class objects equals will always return true and map concludes it as duplicate key.