带有自定义比较器的 TreeSet() 的异常行为

Question

What I've done in the below program is, I've created a TreeSet named "alls" with a custom comparator which compares two list and if they are equal it returns 0 else returns 1. But if I add two same list into TreeSet then TreeSet accepts two same list into it. But it should not contain two same list cuz i've defined a comparator like that.

   List<Integer> a1 = new ArrayList<Integer>() {{add(1);add(2);add(3);add(4);}};
    List<Integer> a2 = new ArrayList<Integer>() {{add(1);add(1);}};
    List<Integer> a3 = new ArrayList<Integer>() {{add(2);add(1);}};
    List<Integer> a4 = new ArrayList<Integer>() {{add(3);add(1);}};
    List<Integer> a5 = new ArrayList<Integer>() {{add(4);add(1);}};
    List<Integer> a6 = new ArrayList<Integer>() {{add(5);add(1);}};
    
    Comparator b1 = (l1,l2)->{if(l1.equals(l2)) return 0; else return 1;};
    Collection<List<Integer>> alls = new TreeSet(b1);
    alls.add(a1);
    alls.add(a1);
    alls.add(a1);
    alls.add(a2);
    alls.add(a3);
    alls.add(a1);
    alls.add(a4);
    alls.add(a6);
    alls.add(a5);
    alls.add(a2);
    alls.add(a1);

When I try to Print my TreeSet(name alls) then the following output shows up:

1 2 3 4

1 1

2 1

1 2 3 4

3 1

5 1

4 1

You can see [1 2 3 4] is inserted into my TreeSet(whose name is alls) twice but it does not get inserted after.

How is this possible? I though TreeSet with my custom comparator doesn't allow duplicates. Also if it allows why not the same elements doesn't get inserted further in my program

Answer 1

Your Comparator doesn't work

Collection<List<Integer>> alls = new TreeSet(b1);
alls.add(a1);
alls.add(a2);
alls.add(a3);

Your implementaton sorts two different list with the second always bigger due to return 1 so your Tree looks like :

a1 <- a3 -> a2

Now let's add a1 :

Collection<List<Integer>> alls = new TreeSet(b1);
alls.add(a1);
alls.add(a2);
alls.add(a3);
alls.add(a1);

a1 <- a3 ? a1 -> a2 RESULT a1 is bigger
a1 <- a3 -> a2 ? a1 RESULT a1 is bigger
a1 <- a3 -> a2 -> a1

Your tree has now duplicated a1.
SOLUTION : Use a correct Comparator mentionned in comments

Answer 2

Try this:

        Comparator<List<Integer>> b1 = (l1, l2)->{

        if(l1.equals(l2)) {
            return 0;
        } else {
            if(l1.isEmpty())
                return 1;
            if(l2.isEmpty())
                return -1;

            int i = 0;
            while (l1.get(i).equals(l2.get(i))) i++;

            if(l1.get(i) > l2.get(i))
                return 1;
            else
                return -1;
        }};

Your comparator doesn't handle the situation when one list "is lower" than the second. Your comparator returns only 0 and 1. I think that TreeSet uses binary search or any faster method to find an object and check if is a particular object is a set. Methods like that need to have clearly defined whether an object is greater or lower than others. If TreeSets use primitive comparing by equal object with each in the set your comparator would be working. In sorted structures are more complex and better methods to find an object.