Can Collections.sort() take treeSet as first argument? If not why?-CodePudding

I have my 'Person' class as follows -

public class Person implements Comparable<Person> {
    private int marks;
    private String name;

    Person(int marks, String name) {
        this.marks = marks;
        this.name = name;
    }

    public int getMarks() {
        return marks;
    }

    public String getName() {
        return name;
    }

    @Override
    public String toString() {
        return "Person: Marks = "   marks   ", Name =  "   name;
    }

    @Override
    public int compareTo(Person person) {
        return marks > person.marks ? 1 : -1;
    }
}

Now in main method I have created another comparator with opposite sorting -

Comparator<Person> comparator = new Comparator<Person>() {
            @Override
            public int compare(Person p1, Person p2) {
                int marks1 = p1.getMarks();
                int marks2 = p2.getMarks();
                return marks1 > marks2 ? -1 : 1;
            }
        };

Now, I create a TreeSet -

TreeSet<Person> treeSet1 = new TreeSet<>(List.of(
                new Person(67, "Himani"),
                new Person(73, "Hasani"),
                new Person(21, "Rohini")
        ));

Now, I try passing treeSet as an argument to Collections.sort() -

Collections.sort(treeSet1, comparator);

Here, I get the following error -


Required type                         Provided

list:List<T>                         TreeSet<Person>
                              
c:Comparator<? super T>              Comparator<Person>

As far as I can deduce from the error - Collections.sort() with comparator can only be used on List and not Set.

Am I correct? If not where am I going wrong? If yes why am I correct?

CodePudding user response：

The first argument to Collections.sort is of type List<T>. So you can only pass List objects to it, not Set objects, or anything else.

This is sensible. The reason is that Set objects are of two types.

There are those that are, by their nature, always unsorted, such as HashSet. It doesn't make sense to sort these.
There are those that are, by their nature, always sorted the same way, such as TreeSet. It doesn't make sense to sort these, because they're already sorted.

I think maybe what you want is to take your TreeSet and have it sorted using a different Comparator. To do that, you'll have to make a new TreeSet with your new Comparator, and copy the entries across from the old set using addAll.