What does return value -1, 0, 1 means exactly in Java collection sort with Comparator interface compare method?

user_4365711 0
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What I understand
If the return value is 0, then there is nothing to do. If it is -1, then the two values will be swept. If we want to change the order (asc/desc), just change the comparison operators.
What I found on the websites
The return value will be -1, 0 or 1 as the first argument is less than, equal to, or greater than the second.
What I want to know
Why do we need the return value 1? I tested my program without return value 1 and it sort correctly.

The following program sort students objects by grade with ascending order. If the sorting order is wanted to change, change the comparison operator.

import java.util.*;
public class HelloWorld {
  public static void main(String[] args) {
    Student s1 = new Student("JJJ", 5);
    Student s2 = new Student("DDD", 2);
    Student s3 = new Student("RRR", 4);
    Student s4 = new Student("CCC", 4);
    Student s5 = new Student("GGG", 3);
    Student s6 = new Student("JJJ", 1);
    List < Student > ls = new ArrayList < > ();
    ls.add(s1);
    ls.add(s2);
    ls.add(s3);
    ls.add(s4);
    ls.add(s5);
    ls.add(s6);
    for (Student s: ls) {
      s.print();
    }
    System.out.println("=====================");
    ls.sort(new Comparator < Student > () {
      @Override
      public int compare(Student s1, Student s2) {
        if (s1.grade < s2.grade) return -1;
        // else if (s1.grade > s2.grade) return 1;
        else return 0;
      }
    });
    for (Student s: ls) {
      s.print();
    }
  }
  public static void printLs(List < Student > ls) {
    for (Student s: ls) {
      s.print();
    }
  }
}
class Student {
  String name;
  int grade;

  Student(String name, int grade) {
    this.name = name;
    this.grade = grade;
  }

  void print() {
    System.out.println(this.name + " - " + this.grade);
  }
}
comparator data-structures collections sorting java
2 Answers

If it is -1, then the two values will be [swapped]

Not necessarily. It depends on what two elements the sorting code is calling your callback with. It may be that returning -1 wouldn't make it swap them but returning 1 would.

I tested my program without return value 1 and it sort correctly.

You just got lucky with the specific data you were sorting, which happened with the specific algorithm being used by the sorting code to not need your callback to be accurate. With other data, it wouldn't work correctly, because with the else if commented out, you'll return 0 (these two are the same) when they aren't the same because s1 will be after s2.

Don't try to second-guess what the documentation is telling you, or how the specific implementation of sort works. Correctly implement the callback so it returns a negative number, zero, or a positive number depending on how to two elements you're given should be ordered.

user_1572470

I've modified your code to generate more data at the start to demonstrate that your dataset is too small to show the problem.

You can see and run it on Coding Ground. (I'm not affiliated)

Don't rely on an abstracted sort call to always use the same sort implementation. It can call different strategies depending on how big your data set is.

user_29831320
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