TreeSet not adding more than two items (objects)

Question

I have a student object which implements Comparable interface.

class Student implements Comparable<Student>{
    String rollNo;
    double internal;
    double external;
    Student(String rollNo, double internal, double external){
        this.rollNo = rollNo;
        this.internal = internal;
        this.external = external;
    }
    public int compareTo(Student otherStudent){
        return (this.external < otherStudent.external) ? -1 :
            (this.external > otherStudent.external) ? 0 : 1;
    }
}

I am trying to add a set of objects from a list of students. The snippet of the input list is as follows:

U4CSE11150,25,24.5
U4CSE11167,4,13
U4CSE12502,23,34.5
U4CSE13001,27,44.5
U4CSE13002,26,30
U4CSE13003,44,62.5
U4CSE13005,32,30.5
U4CSE13006,24,31
U4CSE13007,27,41
U4CSE13008,34,47
...

I am creating Student object within a while loop, adding it to the TreeSet studentsand printing the size of Treesetalong with the 3rd value of the object (that is external mark). The following is the code.

 while((line = br.readLine()) != null){
                String[] elements = line.split(",");
                rollNo = elements[0];
                internal = Double.parseDouble(elements[1]);
                external = Double.parseDouble(elements[2]);

                Student s = new Student(rollNo, internal, external);
                students.add(s);
                System.out.println("Size: " + students.size() + " Mark: "
 + s.external);
                }

But, when running the program, I am getting the output like follows. It is adding only first two objects.

Size: 1 Mark: 24.5
Size: 2 Mark: 13.0
Size: 2 Mark: 34.5
Size: 2 Mark: 44.5
Size: 2 Mark: 30.0
Size: 2 Mark: 62.5
Size: 2 Mark: 30.5
Size: 2 Mark: 31.0
Size: 2 Mark: 41.0

Can anybody help me out? Thanks in advance.

Answer 1

Your compareTo method is invalid - you return 0 when the compared values are not equal. It should be :

public int compareTo(Student otherStudent){
    return (this.external < otherStudent.external) ? -1 :
        (this.external > otherStudent.external) ? 1 : 0;
}