Java: bound mismatch: error while using comparator

Question

I was trying a small code using Comparator to sort list of student through there roll number. But I am getting compile time error at this line:

Collections.sort(l);

Below my test code:

     package test;

     import java.util*;


      public class StudentUsingComparator implements
                    Comparator<StudentUsingComparator> {

        int roll;
        String name;

        public StudentUsingComparator(int roll, String name) {
                    this.name = name;
                    this.roll = roll;
        }
        @Override
        public int compare(StudentUsingComparator s1, StudentUsingComparator s2) {
                    return s1.roll - s2.roll;

        }

        public static void main(String[] args) {
                StudentUsingComparator student1 = new StudentUsingComparator(10, "ab");
                StudentUsingComparator student2 = new StudentUsingComparator(30, "cd");
               StudentUsingComparator student3 = new StudentUsingComparator(20, "bc");

              List<StudentUsingComparator> l = new ArrayList<StudentUsingComparator>();
               l.add(student1);
               l.add(student2);
               l.add(student3);
               System.out.println("unsorted collection is:  " + l);
               Collections.sort(l);   //m getting error in this line
               System.out.println("sorted collection is:  " + l);
        }
   }

Error message says:

bound mismatch: the generic method sort(List) of type collection is not applicable for the arguments list(). the inferred type StudentUsingComparator is not a valid substitute for the bounded parameter>

Tassos Bassoukos · Accepted Answer · 2014-03-11 14:02:27Z

2

You confused Comparable<> and Comparator<>. Classes that have an intrinsic order should implement Comparable<>, then you can use Collecections.sort(l). When you use Collections.sort(l,c), c should implement Comparator<>, which defines an external ordering that is different and separate from the internal.

answered Mar 11, 2014 at 14:02

Tassos Bassoukos

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Comments

piet.t · Accepted Answer · 2014-03-11 14:08:02Z

Your code is mixing two concepts:

You can define a Student class that implements Comparable, then you can use Collections.sort(l) to sort a List<Student> l.
In addition to your class Student (could be Comparable or not) you define an extra class implementing Comparator<Student> and use Collections.sort(l,c) with a List<Student> l and a Comparator<Student> c.

While it is possible in theory to mix the two cases (like you did) and make Student implement Comparator<Student> (and then continue along the lines of 2.) this is a total no-go because either your objects have a natural order - then you should go with Comparable and case 1., or they don't, then defining the order should go to a separate Comparator-class.

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