Why does TreeSet count duplicate elements by HashSet key values ​​and not by the keys themselves?

Question

Java

I am new to Java. I am improving my knowledge of the language on codewars.com. Performing one of the tasks, I encountered a strange thing: I want to implement a priority queue in Dijkstra's algorithm, and for time optimization I chose such a data structure as TreeSet .

import java.util.*;

public class TestTreeSet {
  public static void main(String[] args) {
    int[][] arr = {{1, 2, 3, 4, 5, 6}, {4, 9, 1, 2, 6, 0}};
    HashMap<Integer, Integer> d = new HashMap<>();
    for (int i = 0; i < arr[0].length; i++)
      d.put(arr[0][i], arr[1][i]);
    
    TreeSet<Integer> s = new TreeSet<>(new Comparator() {
      public int compare(Object o1, Object o2) {
        return d.get((int) o1) - d.get((int) o2);
      }
    });
    for (int i = 1; i < 7; i++) s.add(i);
    System.out.println(s); // [6, 3, 4, 1, 5, 2]
    
    int k = 6;
    if (s.contains(k)) {
      s.remove(k);
      d.put(k, 2);
      System.out.println(s.add(k)); // false
    }
    System.out.println(s); // [3, 4, 1, 5, 2]
  }
}

Why (in my situation) does the TreeSet consider duplicate elements based on the key values ​​of the HashSet ?
Is it possible to do the classic prohibition on duplication of elements (keys of HashSet ), but keep sorting by key value? Thanks in advance)

Answer 1

Because that's how you defined the comparator. The compare method has the following contract:
returns

• negative value if o1 is less than o2
  
• positive value if o1 is greater than o2
  
• zero if o1 is equal to o2.
  

Actually, there is no duplication by HashSet keys here - this is the work of your comparator.
And the fact that the result of the work of the comparator contradicts the result of the work of the equals method is a programmer's mistake, from which Java cannot warn in any way.
After making the call, you yourself made it so that , that is, you yourself formulated the statement that 3 == 6. And TreeSet, of course, believes in this, why would it need to double-check you with a call to equals? This is not a hashcode, which is subject to collisions - it is a comparator, which by its nature should not allow collisions. UPD: As for your goal - the implementation of a priority queue - why did the ready-made PriorityQueue class not please you? d.put(k, 2);compare(6, 3) == 0