Java PriorityQueue with custom comparator

Question

I use PriorityQueue and my own Comparator, but somehow the final results are not always good. I have to sort by middle class than name than id.no. In the end, he should return the names remaining in the ordered queue. The other names are beautiful, but they are not in order. Input (name, class avg, id.no):

add John 3,75 50
add Mark 3,8 24
add Shafaet 3,7 35
poll
poll
add Samiha 3,85 36
poll
add Ashley 3,9 42
add Maria 3,6 46
add Anik 3,95 49
add Dan 3,95 50
poll

Expected Result:

Dan
Ashley
Shafaet
Maria

My result:

Dan
Ashley
Maria
Shafaet

Could you help me find the problem? Thank you in advance!

class StComp implements Comparator<Students> {
        @Override
        public int compare(Students st1, Students st2) {
            if (st1.getCgpa() == st2.getCgpa()) {
                if (st1.getName().equals(st2.getName()))
                    return st1.getId() - st2.getId();
                else
                    return st1.getName().compareTo(st2.getName());
            }
            else
                return (st1.getCgpa() < st2.getCgpa()) ? 1 : -1;
        }
    }

    StComp stComp = new StComp();
    PriorityQueue<Students> pq = new PriorityQueue<Students>(2, stComp);

Answer 1

Your Comparatorcorrect. The problem is that you are most likely traversing the list using it Iterator. The PriorityQueuedocumentation states:

, (), .

PriorityQueue , :

while (!pq.isEmpty())
    System.out.println(pq.poll().getName());
}

, .

---

, , PriorityQueue. , PriorityQueue , .

PriorityQueue , Comparator, , :

Student[] students = pq.toArray(new Student[pq.size()]);
Arrays.sort(students, new StComp());
for (Student s : students) {
    System.out.println(s.getName() + " " + s.getCgpa() + " " + s.getId());
}

- Collection , PriorityQueue, :

Collection<Student> temp = new LinkedList<>();
while (!pq.isEmpty()) {
    Student s = pq.poll();
    System.out.println(s.getName() + " " + s.getCgpa() + " " + s.getId());
    temp.add(s);
}
pq.addAll(temp);

---

:

public class Main {

    public static void main(String[] args) {
        PriorityQueue<Student> pq = new PriorityQueue<>(new StComp());
        pq.add(new Student("John", 75, 50)); // Student name, grade average, id
        pq.add(new Student("Mark", 8, 24));
        pq.add(new Student("Shafaet", 7, 35));
        pq.poll();
        pq.poll();
        pq.add(new Student("Samiha", 85, 36));
        pq.poll();
        pq.add(new Student("Ashley", 9, 42));
        pq.add(new Student("Maria", 6, 46));
        pq.add(new Student("Anik", 95, 49));
        pq.add(new Student("Dan", 95, 50));
        pq.poll();

        // Not guaranteed to be in priorty order
        System.out.println("Using PriorityQueue Iterator, may not be in the correct priority order.");
        for (Student s : pq) {
            System.out.println(s.getName() + " " + s.getCgpa() + " " + s.getId());
        }

        // Correct order, but removes from the Priority Queue
        System.out.println("\nIterating until empty using PriorityQueue.poll(), will be in the correct order.");
        while (!pq.isEmpty()) {
            Student s = pq.poll();
            System.out.println(s.getName() + " " + s.getCgpa() + " " + s.getId());
        }
    }

}

(, )

public class Student {

    private double cgpa;
    private String name;
    private int id;

    public Student(String name, double cgpa, int id) {
        this.name = name;
        this.cgpa = cgpa;
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public int getId() {
        return id;
    }

    public double getCgpa() {
        return cgpa;
    }

}

StComp ( )

public class StComp implements Comparator<Student> {

    @Override
    public int compare(Student st1, Student st2) {
        if (st1.getCgpa() == st2.getCgpa()) {
            if (st1.getName().equals(st2.getName())) {
                return st1.getId() - st2.getId();
            } else {
                return st1.getName().compareTo(st2.getName());
            }
        } else {
            return (st1.getCgpa() < st2.getCgpa()) ? 1 : -1;
        }
    }
}

( , , Iterator)

Using PriorityQueue Iterator, may not be in the correct priority order.
Dan 95.0 50
Ashley 9.0 42
Maria 6.0 46
Shafaet 7.0 35

Iterating until empty using PriorityQueue.poll(), will be in the correct order.
Dan 95.0 50
Ashley 9.0 42
Shafaet 7.0 35
Maria 6.0 46

Answer 2

Java 8, Comparator :

Comparator.comparingDouble(Students::getCgpa)
    .thenComparing(Students::getName)
    .thenComparingInt(Students::getId)

Java , . , . :

, c S, , c.compare(e1, e2)==0 , e1.equals(e2) e1 e2 S.

, , ( ). , ( ) c ( ), S. , c S, , ( ) "". , ( ) ( ), equals.

( , , .)

@Override
public int compare(Students st1, Students st2) {
    int comparison = Double.compare(st1.getCgpa(), st2.getCgpa());
    if (comparison == 0) {
        comparison = st1.getName().compareTo(st2.getName());
    }
    if (comparison == 0) {
        comparison = st1.getId() - st2.getId();
    }
    return comparison;
}

, Students equals:

@Override
public boolean equals(Object obj) {
    if (obj instanceof Students) {
        Students other = (Students) obj;
        return Double.compare(this.getCga(), other.getCga()) == 0
            && this.getName().equals(other.getName())
            && this.getId() == other.getId();
    }
    return false;
}