why we need to return 0 when Comparator.compare become equal

I understand that when implement compare method of Comparator interface we need to return

• +1 if o1 > o2
• -1 if o1 < o2
• 0 if o1 == o2

my question is why we need to return 0 when both equal? what is the use case or where it being used? if we considered sorting when o2 greater than to o1 or o2 equal to o1 does not change it place. can anyone come explain practical use case for this?

Java documentation says

Compares its two arguments for order. Returns a negative integer, zero, or a positive integer as the first argument is less than, equal to, or greater than the second.

Does this mean return -1 or return 0 has same impact?

zero, or a positive integer

 @Override
    public int compare(Test f1, Test f2) {
        if (f1.getId() > f2.getId()) {
            return 1;
        } else if (f1.getId() < f2.getId()) {
            return -1;
        } else {
            return 0;
        }

    }

When you are sorting, -1 and 0 essentially have a very similar impact on the ordering of the sorted list as items where the compareTo evaluate to 0 will just be grouped together.

You would "practically" use this comparison in other scenarios, such as where you may not want to duplicate the adding of complex objects to a list (yes, you could also achieve this scenario through the use of a set as well).

Say we have an object Book as follows:

import java.util.Comparator;

public class Book implements Comparable {

  String isbn;
  String title;

  public Book(String id, String title) {
    this.isbn = id;
    this.title = title;
  }

  String getIsbn() {
    return isbn;
  }

  String getTitle() {
    return title;
  }

  @Override
  public int compareTo(Object o) {
    return Comparator
            .comparing(Book::getIsbn)
            .thenComparing(Book::getTitle)
            .compare(this, (Book) o);
  }

  @Override
  public  String toString() {
    String output = new StringBuilder()
            .append(isbn).append(":").append(title)
            .toString();
    return output;
  }
}

Here we have overridden the compareTo of book to create a custom comparison that first checks the books isbn and then it's title.

Say (for example) you have a library, that has books in it. You may want to prevent your user from adding duplicate books in that library ...

public class Library {

  public static void main(String [] args) {
    List<Book> library = new ArrayList<>();
    library.add(new Book("9780593098240", "Children of Dune"));
    library.add(new Book("9780593098233", "Dune Messiah"));
    library.add(new Book("9780441172719", "Dune"));
    // Just to show the sorting, based on multiple attributes.
    Collections.sort(library);
    System.out.println("Books in library: " + Arrays.toString(library.toArray()));

    // You would obviously have some code for entering a book here, but easier to just create the object for an example. 
    Book newBook = new Book("9780593098240", "Children of Dune");
    for (Book bookInLibrary : library) {
        if (bookInLibrary.compareTo(newBook) == 0) {
            System.out.println("We already have that book in the library.");
            break;
        }
    }
  }
}