ADDITIONAL SYSTEM INFORMATION :
Window 10
Java 17, Java 20

A DESCRIPTION OF THE PROBLEM :
Add two student objects same id to a TreeSet

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Step 1: I have a student class with structure: Integer id, String classCode, String fullName, double mark
Step 2: Init a student list have 5 students
Step 3: Create a TreeSet with Comparator compare id
Step 4: Add all 5 students to TreeSet
Step 5: Edit student 1 -> Change id
Step 6: [BUG] Add student one more time

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
Expected result: Don't add student 1 again to Set
ACTUAL -
Actually result: Set have 2 student with same id

---------- BEGIN SOURCE ----------
public class Student {

    private Integer id;
    private String fullName;
    private String classCode;
    private double mark;

    public Student(Integer id, String fullName, String classCode) {
        this.id = id;
        this.fullName = fullName;
        this.classCode = classCode;
    }
    public Student(Integer id, String fullName, String classCode, double mark) {
        this.id = id;
        this.fullName = fullName;
        this.classCode = classCode;
        this.mark = mark;
    }

    public Integer getId() {
        return id;
    }

    public void setId(Integer id) {
        this.id = id;
    }

    public String getFullName() {
        return fullName;
    }

    public void setFullName(String fullName) {
        this.fullName = fullName;
    }

    public String getClassCode() {
        return classCode;
    }

    public void setClassCode(String classCode) {
        this.classCode = classCode;
    }

    public double getMark() {
        return mark;
    }

    public void setMark(double mark) {
        this.mark = mark;
    }

    @Override
    public String toString() {
        return "Student{" +
                "id=" + id +
                ", fullName='" + fullName + '\'' +
                ", classCode='" + classCode + '\'' +
                ", mark=" + mark +
                '}';
    }
}

public class TreeSetDemoTest {

    public static void main(String[] args) {

        TreeSet<Student> tSet = new TreeSet<>((s1, s2) -> Integer.compare(s1.getId(), s2.getId()));
        Student st1 = new Student(1, "Nguyen Van A", "FJB01", 6);
        Student st2 = new Student(2, "Nguyen Van B", "FJB01", 8);
        Student st3 = new Student(2, "Nguyen Van C", "FJB02", 4);
        Student st4 = new Student(4, "Nguyen Van D", "FJB02", 10);
        Student st5 = new Student(5, "Nguyen Van E", "FJB03", 8);

        System.out.println(tSet.addAll(List.of(st1, st2, st3, st4, st5)));
        st1.setId(69);
        tSet.add(st1);
        tSet.forEach(System.out::println);

        for (Student student : tSet) {
            System.out.println(student == st1);
        }
    }
}
---------- END SOURCE ----------

FREQUENCY : always